ternrorci fu'ivla

x1 is the co-parent of x2 with parent x3.

x1 is the baby momma/daddy (slang, but connotatively neutral, term) of x2 from the perspective of x3. See: "malternrorci".

In notes:

malternrorci
x1 is an absent or deadbeat co-parent of x2, the 'primary' parent of whom is x1.