Instead of being able to choose only one, it would be way better if you still able to get the rest of them somehow (on the same tutorial or maybe on a more advanced progression) , so you don't have to worry about having to pay 3k every time you want to change your role on a mission.
Maybe you're an OCA main but you need to swap to Sniper for a certain moment, so you actually have to pay 3k for a weapon that you will not use that much at all, but you really need it on that situation.
For frequent players it may not be an issue at all because they have a lot of money already or they just play the game everyday and get a good cash flow, but for players that just started playing the game and for those that don't play it that frequently paying 3k for each time they want to play a different role may really hurt their pockets, so IMO a change like this it would be really beneficial for incoming new players and casual players in general.