The way I see it… simply giving you 2 5* heroes to match those costume pulls would not exactly be fair, because you did those pulls with the higher chances for the 5* costume, and lower gem cost. Re-doing those same pulls with the adjusted odds of the re-designed costume chamber would likely mean you would not get 3 5* this time, plus you would pay the higher cost. (But on the other hand you could’ve gotten the HotM.)
I’m not sure there’s a realistic way to “adjust” previous pulls in a fair way, and I would not expect that to happen. Paying the gem cost difference alone in exchange for the missing heroes would not be enough to achieve fairness, because you had the higher chance of 5* at play too and I don’t see how that can fairly and realistically be adjusted for.
Don’t take this the wrong way, I don’t begrudge you your heroes, I hope you get them actually… it’s just that this way wouldn’t be “fair”, just my 2 cents. And btw, congrats on pulling three 5* costumes, that’s probably more than most people managed, myself included. I’m sure the 2 missing heroes will show up in time… put some TC20s to work maybe?