Optimal depends on the rest of the list. IMO, with the nerfing of faith, the optimal setup might just be 2x flamers because it's the cheapest. But with the amount of melta fire in your list (just 3x doms and that's it) I think your BSS's need some melta power. The MM, flamer, combi-flamer seems situational to me, in games where the enemy comes to you, or when the enemy lacks the firepower to take out your rhinos when they're taking multi-melta pot shots from 24", it's gravy. But as you point out, against certain armies like IG if you don't close range you're dead, so that MM is going to be wasted. So I would lean towards melta/melta/combi-flamer. Also think about melta/flamer/combi-melta, unless your meta is entirely MEq and the melta is worth a lot more to you than the flamer.