I, personally, don't mind the stares that come with PDA when we three are out of our local area (I'm a bit of an exhibitionist that way
) BUT, when we are out locally, I am careful to PDA only with MrS (my legal husband, known to the community) and only with Dude (or Lotus) at the level of "affectionate friend".
So, it seems to me that you are not comfortable "drawing attention" to the fact that she is with two guys - but they don't care. OK, that seems perfectly reasonable to me. Since you are OK with PDA with her while you are out UNLESS she is (or has recently) displayed PDA with him...and seems to be reasonable on the topic since you discussed it with her in more detail....how about including him in that conversation as well? If they are willing, you all could decide who gets to "play being her bf" for a given outing. You get to be comfortable not "looking like" poly in public and she gets PDAs from you with the other SO present...
What's your relationship like with your metamour? Would he be willing to sacrifice a little PDA to help you two out? Seeing as he is more "forward" he probably feels less limited that you do - but does he really NEED to be able to PDA or does he just do it automatically, not realizing that your response is to shut down your own PDA instincts?
(My Dude is completely clueless as to why other people respond to situations in a different way than he does...but if you explain it to him he is usually willing to accommodate other people's desires...)