Gerald,
Thanks for your comments.
I agree that Frame 3 is not the best structural arrangement, but if you look at a section through the fuselage at Frame 3 with the gear retracted and pipes installed, you will see that there is no space available to close out that frame.
In order to try to ensure adequate structural strength, I am planning on making Frame 3 out of 3 layers of 1/8" 5-ply. Two layers will have the cut-outs necessary for the ribs and retract plate, but the 3rd frame layer will omit these cut-outs in order to eliminate any potential weak points. I may even go to 4 layers, 2 of each. That would make that frame 1/2" thick. See frame layouts below.
If you can think of a better way please let me know.
Paul