I've been following this thread with great enthusiasm because it is addressing the persistent mysteries of the ANH studio models, with input from the best and most knowledgable (Beaz, Jason, etc.) and those with priveledged information.
I can understand why Blue 5 has doubts about the Red 3 hero - it stands apart for many different reasons, not all fully explained yet.
I think the evidence points to the fact that it definitely existed in ANH, but what I've never been able to reconcile is its colorful paint job dating back to 1976(?). Yes, two views of it do appear in "The Art of Star Wars" first published in November of 1979 (ESB was of course fully underway by then) although it does take time to prepare books for publishing, but I'm not fully convinced that it had all those funky colors painted during ANH.
I tried "training" myself to spot the heroes in the film footage, taking into account the color washouts and lighting problems discussed earlier. I can now spot the Red Five and Red Leader ships with confidence, as well as most others.
However, the way the Red Three hero appears in the film (ANH) does not seem to match published pictures of it, no matter how hard I try to stretch the imagination. There simply seems to be no hint of the yellows, greens, etc, or any matching markings (except for the three stripes, of course). Even the excellent and new photo posted by Beaz (with D. Muren) seems to show much more subdued colors.
And those clips which seem to show at least some red wing stripes oversprayed into the chamfer and front of the wing - they do not seem to match the published pictures, even before they added the "landing gear doors".
Red Three is now one of the most colorful hero models, but I still believe that it was (at least partially) repainted for ESB. It's original character (Biggs) was lost in ANH, yet it is the only hero model which was filmed in all three movies which originally was meant to represent a destroyed ship. And let's not forget that different lower nose aft section - the only X-Wing to have that, and yet a feature copied on the MPC and Fine Molds kits.
P.S. - can we come to an agreement as to how to more accurately name the Y-Wings? "Triangles" (the one with four dark triangles on each engine apexx) was Gold Five ("Pops"), a blown-up pyro whose engine went flying off. "TIE Killer" was Gold Leader (Dutch), "Blue/Gray" was Gold Two (Tyree), and the "MoM" Y-Wing, with the brownish-green stripes on the engines and silver droid with black markings and a thin red stripe, has unofficially been named "Gold 3" in the Fine Molds instruction sheets. By studying film clips and pictures of that ship, I am convinced that it appears at the end of ESB. The markings seem to match. Lorne Petersen refererred to it as an "unexploded pyro" but they added lights to it and a few more streaks and dirt.
Wish we had a better name for "Red Jammer", and that leaves the "Bill George" Y-Wing with the distinctive R5 droid.
P.P.S. - previous ref. to "Mr. Ladd" - that would be Mr. Alan Ladd Jr. who is still alive and well! Not his deceased actor father! And, since both "leaders" are missing, wouldn't it be plausible that both the Red Leader and Gold Leader hero models were retained by their ultimate creator, Mr. Lucas?