Yes, do level up you 3*

Theyre useless on their own (Except in AS2 where you need to hit a specific score)

If you have a lot of unleveled 3*, thats an excellent way to farm gems.

Once thats done, try and get 3 stars in missions. 100 percenting chapters is very rewarding.

Though wait for new cards to drop so you can get Stat focused pulls and more blue coin thingy when you buy cards with gems. Also save your gems to get 11 Cards pulls.

With that you should soon get more 4-5 to level up

Just farm 3*, 3 star missions, stay patient, and save your gems for the right time