Rank up 7 star R3 ?

Eru_iLaa

Could you help me please for rank 3?

Magrailothos

Several good options - my top three would be:
Photon
Beta Ray Bill
Kushala

If you're into Battlegrounds then Photon is a general dual threat; BRB is a pure attacker. Kushala is primarily an attacker, and one of the best Serpent counters in the game.