Let's consider, firstly, who gets the initiative.
That's Luke, obviously. And what's he going to do? Well, he could target one of them quickly - probably Dooku, the weak link when it comes to Force powers. How easy would it be for him to kill or incapacitate him? Take a look at how easily Sidious chokes him through a hologram; what if Luke just does that, but ragdolls him far instead, slamming him against a wall and putting him out, at least for the moment?
Or he could do this to Vader by taking advantage of his life support systems, casting emerald lightning, which Vader would not expect from a Jedi.
Once it's down to 2 vs. 1 it's not even a serious question.
If he really wants to take them all on at once, they probably can't hurt him from a distance; he tanked UnuThul's TK in a pretty absolute manner, and could presumably resist Palpatine's lightning, which outmatches Galen and Dooku's combined demonstrated power output based on its feats. Luke could instead blitz them; then, take a look at how Windu, Dooku's equal or superior, was utterly incapable of doing much against Sidious before he sank into vaapad; that speed disparity is even greater here. Will he really contribute that much? Would the other two, who are potentially below Dooku in sabers? Numbers also don't mean that much if there's no coordination; Vader and Galen might have some, but they obviously aren't beating him together, and Dooku has basically none with them.
I think their best shot is to have Galen and Dooku spam lightning while Vader either hurls projectiles or goes in close. Still, Luke could easily just throws their lightning back at them, beat them on the draw, or continue to move and defend himself while being attacked; he can defend against The Hidden One's lightning and TK at the same time, for example. Even by the beginning of Dark Empire he can absorb an AT-AT's laser cannons via Force barrier, so he may be able to do this while targeting them one at a time.