Team compostion

Probably the best servant you got out of the three would be Nero. Emiya would be a great investment for the long run but he would be very taxing at the start since he would be relying a lot on his strengthening quests and from what I heard he should be getting another strengthening quest by next year. If you're F2P I suggest you would like to make a team comp of hard surviving servants with OG Cu Chulainn then for crit, buster and supporting casts it's highly recommended to pair nero up with both Hans and Shakespeare. Hope this helps. :D

While they are all great servants, nero is easiest to fit in an arts team. However, emiya is arts and carmilla is quick but with buster nps. Furthermore, carmilla is a damage going up against riders, so your typical caster supports are difficult.

3 of your gold servant considered pretty broken. Pretty much if they're lvled decently you can clear FGO without too much difficulty.

Magikarp, immortal saber and Female slayer ( most of FGO servant is female )

Hans is a great low star support. High level master still using him on a hard fight or daily purposes. Work for any team composition.
Mozart can serve Emiya and Nero instant Crit and Art steroid for a turn with relatively short CD.
Shakespeare pretty much dependable if you're using him with Emiya. All of his kit can supp Emiya.