What servants should I focus on

EMIYA & Arash have an AoE NP, Robin a ST NP.

It depends on your need of a ST or AoE NP. Arash is more of a farming unit.

The only ST Np in your archer class is Robin Hood since Emiya and Arash are AoE. It really depends on what kind of team you would like to make with these servants since they have different uses.

Emiya is great for a arts team with his triple arts deck.
Robin Hood with his very high damage ST Np will be very helpful against tanky bosses.
Arash makes a great wave clearer with his high damage AoE Np and he will die after use if you don't want him anymore.

You should prioritize on leveling Robin Hood at the moment because he will be very useful against some bosses. After you are done with Robin Hood, you should start leveling Emiya since he will be strengthened later on in the game.

*small correction. EMIYA and Arash have AOE nps, not ST ones and all have VERY, VERY different uses.

I say do all of them, but focus on which one you need the most atm.

EMIYA should be your lowest priority, as he is very mediocre until he can get hawkeye and projection from his Strengthening Quest. Until then keep him for last.

Arash is a great farmer, as he is easy to get to np5 and his Np hits really hard, at the cost of his life. Level him if you wanna farm everything except lancer dailies.

Robin hood is the best ST archer nuker in the game. Yew bow + sabotage is a great combo, and allows him to push out monstrous levels of damage on sabers and beserkers. Level him if you hate sprigans and chimers