I am wondering, might it be helpful to take group sex off the table for now, just be with each other one on one? Then you wouldn't have an opportunity to compare how he is with you versus how he is with her. You could just ask him if they had a nice time, and leave it at that.
I think it's likely that he had some performance anxiety with you before she entered the picture, hence his occasional dysfunction back then. The fact that he is having normal PIV sex with her is likely making him feel anxious that he give the same to you, and the anxiety about performing with you is what is killing his erection.
I am also wondering if his libido is up to the demands of giving two women PIV sex when they want it? Not every man is capable of pleasing two women that way, not even younger men. I have had two partners who had fairly low libidos, they were both quite happy with sex once or twice a week, and less was not a problem for either. One of them was only 28.
I think it is very wise to stop focusing on PIV sex for now. Give him other ways to satisfy you. Try your best to stay calm even if he can't get an erection, or loses it quickly. I know this is hard, but it is the best way to help him.
I know that this must be painful for you, and please know that you are not alone, I have lived a version of this myself, albeit in a quad, not a triad.
Oh, and maybe he can have his testosterone levels checked? Performance anxiety often goes hand in hand with insufficient testosterone. It is a very underdiagnosed issue, and two of my poly partners deal with this problem, as well as several friends of mine.