So I was messing around with it more and talking to a friend about it. Can anyone tell me if this is correct...
$P(W) = 1 - P(W')$
$ = 1 - \left(\frac{_{13}C_3}{_{15C_3}}\right)^3 $$ = 1 - \left(\frac{^{13}C_3}{^{15}C_3}\right)^3 $
Which comes out to $P(W) = 0.752$
It just seems quite high to me.