x1 is the child of the ex-spouse of x2 by standard x3.
Technically, any child co-parented or generated by x2 and the ex-spouse would count so long as the spousal relationship is terminated; usually/conventionally, x2's own/direct children are excluded from the semantic coverage of this word however. x3 covers the relevant parent-child relationship between x1 and the former spouse, the terminated spousal relationship between x2 and the ex-spouse, and the standard by which that spousal relationship was terminated. This word is a sort of opposite of the "step-" in "step-child".