Once they get to a certain size, it's really hard to separate multiple seedlings growing close together. The roots get tangled around each other and you have to break a lot of them to get the plants apart. The reason you got the plants this way would seem to be that the grower decided it was too much effort to try and separate the individuals, actually. So yes, you can pull them apart but you need to do it as carefully as possible and with as little damage as possible, and then give the plants a period of recovery afterwards in a protected place.
I wouldn't attempt the maneuver though, given the plants are showing signs of stress and looking peaked.
I had a Washingtonia in a large pot for several years (as well as 2 in the ground) and it did okay, not great, despite good conditions and attentive care. In my opinion there are many other palms which would be better candidates for container culture. I realize you already have the plant, so I'm not trying to be all doom and gloom here, just provide a bit of a reality check. Enjoy your Washingtonias while they last, but temper your expectations.
When they seem to have recovered from the wind or whatever made them weak, put the pot out in the sun again. Maybe provide a period of gradual accomodation over the course of weeks, gradually ramping up the sun so there's no shock if the plants came out of a greenhouse or something. But you cannot provide too much sun to your plants, the more the better. They come from an arid climate with seasonal rainfall, and they grow in arroyos (dry canyon beds) where their roots have lots and lots of space to burrow into the sandy soil. They will do best if you water deeply and then wait for the soil to dry out most of the way before watering again.