is the tank tall or very long?
although in a tank that size they may be ok, its still not guaranteed, and if the tank is tall, its probably not the best environment for them anyway. they like to be able to rest on the bottom of the tank and then swim up for air. only asking because my 450L tank is 2ft tall, and i wouldn't put my bettas in there (even if they weren't going to get eaten by my oscars! lol) because its such a distance from bottom to top.
if you decide to move them out of the big tank, then definitely separate them, as bettas are notoriously temperamental and can be fine together for ages and then suddenly turn on each other. you can keep several females together but its recommended to have a minimum of 4 together, as this cuts down on bullying and aggression
personally i wouldn't keep them together even in a REALLY big tank like yours, because i have seen the damage that having a rogue male in with the females can cause (and thats with a juvi!) but thats just my opinion.
hope that helps