You want to use Set rather than SetDelayed
Clear[f]
f[x_, y_] := x + 3 y /. {x -> -x y}
DownValues[f]
(* {HoldPattern[f[x_, y_]] :> (x + 3 y /. {x -> -x y})} *)
Trace will show you the evaluation sequence
f[1, 2] // Trace
Clear[f]
f[x_, y_] = x + 3 y /. {x -> -x y}
(* 3 y - x y *)
DownValues[f]
(* {HoldPattern[f[x_, y_]] :> 3 y - x y} *)
f[1, 2] // Trace
Beware that if x or y are defined before the Set this will fail. For a solution see:
