Can a 24 hr department store lock two of its three main entrance/exits at 10:OO PM. ?  Can a door marked "Exit" be locked?

Ideally they should be left open but it all depends on the occupancy how many people are likely to be in the store after 10.00 pm, till all the doors are reopened, also it the means of escape (MoE) compromised, are any of these doors required as an alternative MoE. A Fire Risk Assessment should have been carried out to decide if closing the doors is acceptable and you would need more information before you could give a positive response .