So for an MMA fighter, ideally one should have three sets of gloves? One for MMA, one for sparing (boxing) and one for bag work? And at the least two sets, one for MMA and one for sparing/bag work?
Yes, definitely. Ideally, you'd have one pair for sparring, one pair for heavy bag work (works for pads, too, esp. for straight boxing and MT training) and one for MMA drilling (such as mat drills, grappling, sprawls, stuff like that.)
You can get away with two sets, but you're cheating your partners when you do that, sometimes. While the level of padding in a glove doesn't reduce it's concussive force, it DOES reduce the amount of damage it causes to the face of your sparring partner.
So, if you're working the heavy bag with the force that you should, you'll start smashing down the padding of your sparring gloves and create flat spots. Which can lead to more bruising on the face. Flat spots make creases at the edges of the glove. This can even cause cuts.
Plus, I see people doing push kicks on the heavy bags with shoes on all the time. Just fucking around, you know? But now someone is taking the same gloves that they did bag work with, getting the shit from these people's shoes all over them, then shoving them in my face. It's just not cool.
So yeah, it's not cheap having three sets of gloves... but it's worth it. And as long as you're using them properly and they're of solid quality, you won't need to make another glove purchase for a while.