Should I choose camila,cherche or beruka for my flier emblem team?

Cherche is the best of them, but I think beruka would fit your team better just to have someone that is bulky enough to tank a hit, since Corrin and Eli mostly want to attack. But that's just my opinion

Camilla and especially Beruka are enemy phase units. It you want them to kill things, you'll have to invest quite a bit.

Cherche is a player phase unit with Brave Axe, who generally won't have a problem killing things even with all the 4* talents (assuming +Atk). I say go for Cherche. If you want to try her without investing any feathers, a 4* Cherche with regular Brave Axe and DB2 will already double for 48 before flier buffs or seals.

If you are planning on running a blade on S!Corrin, you would definitely want hone fliers. The extra 18 damage (36 when she almost inevitably doubles) from the atk/spd buff would be a higher priority than buffing her defenses and giving her 12 extra damage. But if you don’t have it, not much you can do.

As for your question, Beruka and Camilla require heavy investment (multiple 5* sacrifices, especially on Camilla) to really shine, but Beruka could be ok with a relatively budget build if she is already 5*. Cherche would cost the least. Definitely give her death blow and drag back/hit and run (if you don’t have a 4* Clair, go Drag Back) along with either iteration of Brave Axe (+ is ideal, but not totally necessary) and she will hit hard. At a +10 4* max merge, Cherche can hit for 54 damage with a neutral Brave axe, 57 with a +. 5* unmerged pushes those values to 52 and 55. Go with Cherche to have a solid unit that requires little investment for viability.