Chinese word for X chair in a musical group when it isn't 1st or 2nd chair
You can just use 第X席. E.g.2nd violin 5th chair 第二小提琴手第5席. 

You rarely saw it probably because if the x>2, they probably do not feel that proud. And no matter 第3席 or 第9席, even the 副首席 does not mean too much to the non-expert audiences (in China)--the spotlight is only on the 首席第一小提琴手.