Each' will work in general but it's best to use vector operations where possible. Here I use the . operator to apply our function, \t to time both methods. I store their results to r1/r2 to show they are the same:
q)t:([]a:til n;b:til n;c:til n:1200300)
q)\t r1:update d:{x+y+z}'[a;b;c] from t
289
q)\t r2:update d:{x+y+z} . (a;b;c) from t
20
q)r1~r2
1b
q)r2
a b c d
-----------
0 0 0 0
1 1 1 3
2 2 2 6
3 3 3 9
4 4 4 12
5 5 5 15
..
Cheers, Ryan