Thank you for your answers!
The reason for thinking it will be a triad is that I think this is what is most acceptable to everyone at the moment. I do not think they will find somebody else to be in a triad with since they weren't looking for someone new - they're just trying to figure out how they want to continue the relationship that has already started with me. I feel that the risk is far greater that they decide that they don't want poly at all..
I don't mind if they relationship reform in the future to include more people... But I think a triad is what they are most ready for at the moment.