With R5-P8 can you only use one charge/reroll per round?

Or can you use 2 or all 3 in one shot if needed?

The trigger is "while you perform an attack against a defender [...]" Therefore, thanks to the once-per-opportunity rule, you can only trigger it once during that attack. So you get one charge/reroll per attack . (Up to two per round, if you can get a bonus attack.)