After a story about the boards themselves and another one about the design, I now arrived at the point when I am looking at details.
As we saw, the English (style) tracing boards are mostly based on those of John Harris and Frieda Harris seems to have based hers on those of John Harris.
Let us compare John to Frieda Harris and start with the first degree tracing board:

Frieda had a much different style, but it is all there. The huge level, the rough stone, a stone with a lewis (very English), the circle with dot on which lays a Bible on which stands a ladder with the symbols of Faith, Hope and Charity. Three pillars, the sun and the moon. The only thing that I notice is that Frieda Harris added a sword at the bottom left corner. I have no immediate explanation.
The second degree then.

Obviously, Frieda’s tracing board is much more elaborate. There is something that catches my eye. At the bottom left, you see a man guarding the staircase. This is the Tyler in a Masonic setting. That Tyler is present on John Harris’ design and in the Scottish Workings, but missing in Dharma and later co-Masonic tracing boards. This could mean that Frieda Harris did not take the tracing boards of her own organisation as inspiration, but did some further investigation.
Frieda Harris moved the ear of corn and water wonderfully to the centre of the image (look between the pillars for the corn in the image on the left). These symbols have a Masonic significance. Also note the typical mosaic pavement, which appears to be very English. It can be found in both Harrises. For the rest, Frieda’s second degree tracing board does not contain elements that call for further investigation.
On to the third degree then. This is where Frieda ‘goes wild’.

It almost looks like it that Frieda Harris put the actual tracing board in the middle and started to work around that.

As you can see, the 5’s are there (Frieda did replace one by a Heh, the fifth letter of the Hebrew alphabet, numerical value 5. I actually know a John Harris design on which all fives are replaced by Hehs). The entrance to the Temple is present. The tools moved outside of the coffin, but roughly this part is the same. The cypher is that of John Harris, common on English style tracing boards which can also be found on co-Masonic tracing boards.
But when we look to the ‘extra’ parts on Frieda Harris’ tracing board, there are a lot of elements there that are not on John Harris’ tracing board.

The Pythagorean theorem, also known as Euclid’s 47th proposition is quite prominent. I do not know of a third degree connection. Often this is a part of the ‘Past Master’ symbol (of a person who has been ‘Worshipful Master’), but it is not particular to the third degree itself.
What is interesting is that John Harris made more tracing boards, not all in the same style and with similar content. In 1849 he also made this third degree board which includes both the theorem and a pentagram (see below).

This could suggest that Frieda Harris had a look at different tracing boards and from different traditions before she decided what to include.
Then we come to another uncommon element:

Frieda Harris added constellations. I see no third degree significance. You do often see (especially in Europe) the constellations against the walls of lodge rooms, making a walk through the room a walk through time. I have found no example of a tracing board where the constellations are so prominent. I do know one tracing board with a clock on it.

Frieda added Hebrew on a few places. On the left of her design (and the top of the image above), you see the Shin (letter S / numerical value 300).
On the dark yellow panel on the top right there is (from right to left) Yod (I / 10), Heh (H / 5) or Vau (V / 6). Ironically, those are the letters of the Tetragrammaton (HVHY). The two characters are above one of the many skulls and crossbones, which does -to me- does not give an extra clue to their meaning.
On the same panel we see red characters that are not too easy to decipher. My guess is Gimel (G / 3), Peh (Ph / 17) and Dalet (D / 4) or Kaf (K /11).
Then (yellow) Tet (T / 9) and perhaps again Dalet (D /4).
The line of three is something like Thav (Th / 400), Heh (H / 5) and Dalet (D / 4)
And then at the bottom Heh (H /5), Aleph (A / 1) x and another Dalet (D / 4)?
This would lead to something like S – YHV – GPDK – TD – THD – HD
None of which rings bells.
Continuing clock-wise with the next panel, there is another detail for which I have no explanation.

On the next panel follows a multi coloured five pointed star which reminds a bit of the Order of the Eastern Star emblem which has the same colours, but other than there is another reference to the number 5, I am unsure. A pentagram appears on the alternative third degree tracing board of John Harris as we saw above.

Then the detail that got me wondering.

Some sort of magic square of which only the row 3-5-7 (in the middle, bottom to top) has Masonic significance. The square is 15 in total in every direction, it says 2+7+6 | 9+5+1 | 4+3+8
Perhaps the 3+5+7 was the inspiration, even though this is not specifically a third degree element.
Of course, around the coffin there are three times the number 5, which also adds up to 15. Possibly a reference to the story of the degree?
At the bottom there are seven stars (in different styles) referring to seven planets.

Which do not have a specific significance in the degree as far as I know.
Also I have no (third degree) explanation for the (15?) ‘cherub wings’ that surround the image.

So why and from where were these elements taken and what are their significance to the third degree? Was Frieda Harris just artistic? And what would be the reason that the first two boards are somewhat traditional (just not in style) and the third something completely different?
Julian Rees (member of the same organisation as Frieda Harris was) published a wonderfull book about tracing boards. The last mentioned are those of Frieda Harris. Of the third degree design Rees writes:
Apart from the symbols mentioned here on these last three boards, there are many more, and to the knowledge of the writer these have never been adequately researched and explored. Were there to be a fitting postscript to this work, it might perhaps be an invitation to an inquisitive and adventurous reader to undertake this virgin field of research.
I may be inquisitive and adventurous, but as you saw, I have not found many answers. Perhaps the above text can be the first step towards further research.
It would be nice to know if anything was published about these designs. Did Frieda Harris say anything when she presented them to her lodge? Was there a text in an (internal) publication or a secondary source? Let me know when you know anything.