Circuit of constant depth that takes three binary numbers and outputs two binaries? How can I construct -- or better to say what type of circuit can be like this -- a circuit that takes 3 binary numbers (a, b, c) and then calculates two binary numbers (d, e) in a way that a + b + c = d + e The circuit should be of constant depth and furthermore, a, b, c, d, and e are as follows;

a=a_n−1…a_0
b=b_n−1…b_0
c=c_n−1…c_0

d=d_n…d_0
e=e_n…e_0
A circuit which would satisfy this requirement is a carry-save adder.  I'm sure you can find a reference for the gate layout somewhere.