I have a very trivial question to ask and it would be great if someone could help me in this. The statement that '3-point amplitudes' and the location of poles are sufficient to determine any n-point amplitude at tree level is confusing to me. Don't I also need to know 4-point amlitudes, for example in YM theory ? The reason I say this is the 4-point vertex can not be broken down. That is the residues obtained upon putting propagators onshell will also contain 4-point functions. What am I missing ? Thanks for your help in advance.