I want to add a receptacle to a circuit with a traveler
So if this is on a traveler circuit 


Do you have two switches somewhere that controls that circuit ?


I think your logic about what you intend to do is fine. You will have a hot a neutral and a ground. 

One of the things that looks off to me is that normally your switches have the traveler between them and not your luminary box. You would see here Two 4 wire romex's [red to red ]and then your 3 way power romex - perhaps you had two sconce lamp receptacles..