In my experience as a DM, most fast healing or regenerating monsters don't really derive too much benefit from those abilities, unless they're played as hit-and-run attackers. Even a Hydra's fast healing won't be able to keep up with a normal party's damage output per round. It's just as if the monsters have some additional HP, with the amount depending on how quickly you can take them down.
Just focus on one at a time, disable or slow the other foes by applying some means of battlefield control, and you should be fine in most cases.
Regeneration can get nasty if you don't have the damage type required to bypass it ready to hand. But even regenerating monsters can often be knocked unconscious quite easily, after which you can stick their heads in a bucket of water or whatever.
So the advice I'd give is just: deal more damage, more quickly, and in a more focused way.
Sure, you can also get fancy and cast Grimwald's Graymantle, or attack ability scores, or something. Do note that ability damage isn't as easy to deal as HP damage, though. For a still incomplete but already quite extensive resource on ability damage, drain, and penalties, see
here.
