Answer
Two 2+ and one 3+.
Work Step by Step
Oxidation numbers:
O: -2, Y: +3, Ba: +2
For copper (one is +2):
$0= +3+2\times2+2+nOx_{Cu}\times2-2\times7$
$nOx_{Cu}=+5/3\rightarrow wrong$
For copper (two are +2):
$0= +3+2\times2+2\times2+nOx_{Cu}-2\times7$
$nOx_{Cu}=+3$
