Usually you can LB against all three bosses. By the time you get to the first boss it's either full of almost full. You can't charge past the first bar except against the 3rd boss so use it as soon as it's full. Then it will likely be full or almost full by the time you get to the second boss so use it when it's full again. If the boss has phases though you want to use it at the right time. Like some bosses will start summoning ads at 50% Hp or stuff like that. In that case use it right when the boss starts summoning the ads for instance.

On the 3rd boss try to wait until the 2nd bar is full but if it's clear the boss will die before it fills then just use LB1