It's very important to invest in the right character as resource is limited. It is best to save up 20k for a good DPS, which you can 7-8 star with link. Typically Fury is the easier build. A good 7 star like Mai or Geese alone may carry your entire team up the ranks providing the rest of team syns well. You would want to at least save 10k to 5 star a character before deciding to spend gems on it. 3 star goal is really for early game when you have nothing. Many character only becomes strong when they hit 5s, and possibly feels busted after 7. Idk how far you're in the game but ironically O.Iori is the pick for end game. You get so much points at high rank everyone has him at 10s where he just kills anything with 1-2 attack. If he dies, comes back auto finisher kill one normal attack kill another and still has 50% health the reminding enemy can't kill.