Best use of surplus 3-star servants?

It actually doesn't matter that much as either way, both exp and mana prisms are farmable. What you need more, pick that.

I usually burn them since MP is harder to farm than EXP. It is my personal rule to never use 3 stars as EXP anyways.

Burn them, the ratio of QP spent to exp gained from silver and bronze cards is really really bad.