I was just wondering on how much you guys change to another merc while in a game (mainly pubs) because I often find myself sticking to one merc for whole games. Of course it depends on the game conditions and what merc(s) you feel like playing as well as which mercs you do best with.
Changing mercs in game
Use the F keys(F1, F2, F3) by default. I have mine set to the 1, 2, and 3 on the numpad because it’s easier to reach in need since I’m on a laptop where I need to press and hold an additional key to enable the F-keys.
[EDIT]: I was half asleep when I wrote this and so didn’t fully understand the thread. I barely change mercs unless my team is in dire need of something in particular, usually I stick to one thing and I’m able to make it work no matter what.
For me it depends a lot, what comes about public games.
I love sniping in Chapel, but if we already have one Vasilli going to the action, I usually change to another merc. Same goes with another mercs: I’m not going to be third Bushwacker or 4th Skyhammer and the list could continue.
Though sometimes I end to play full round with the same merc, if team situation is good enough. Often I’m the only Phantom in the team, so I have free space to play with him full rounds. 
Even when Sawbonez isn’t my favorite merc, I sometimes switch and play with him, for being able to support my team. This is definitely a thing that I will continue with Phoenix later and swap between Phantom and Phoenix, if needed.
Every time I die.
F1 - Bushwhacker
F2 - Bushwhacker
F3 - Also Bushwhacker
There have been some times I really needed Also Bushwhacker to get that one push the team really needed to win the round and field goal the homerun high score touchdown or whatever.
I usually play what the team needs. If there’s no medics, I play medic, if there’s no engineer, I play engineer, if there’s both, I usually play what I have in third slot aka what I want. Though, usually the balance of team’s classes don’t change much during the game so I don’t have chance to play more than one class either
I prefer to start my merc selection in the loading room, gauging what my team is bringing to the table and rounding out wherever they’ve left weaknesses. The vast majority of the time, this means bringing an objective specialist, a medic, and then a third class - something I’ll enjoy. Most of the time, I get stuck playing one of the first two because we’ve got two snipers on the team or something. If you want to win, don’t count on your team too heavily. I know that sounds counter-intuitive and almost certainly doesn’t apply to organized games, but public comp queue is not always filled with people who want to play a competitive team game.
I constantly change depending on what stage we are on the map and what map / side ( attack or defense )
Chapel defence: Skyhammer or Arty to start with then if they get to the last stage switch to Aura ( or if there are too many skyhammers / artys i will choose Aura all the way through )
Chapel Attack: Bushwhacker to start with maybe even all game. Unless again there are too many Engis i switch to Aura / Or if they are set up behind that rock at the end Skyhammer / Arty to destroy the Healing stations and clump kill them.
Same goes for the other maps really. My main 3 Merc setup is Aura/Skyhammer/Bushwhacker. ( changes due to daily missions ofc. )