Jimbojango,
I am sorry if I have given you wrong information, but I addressed that scenario with the NRMP representative directly, one where you ranked a 3 year spot and did not rank a prelim program in your supplemental list.
He said that in that case you will get an email that says that you did not match in a preliminary program or something like that, but you will definitely be aware that you did not match in one.
I had the same situation for one of my programs and that is how I found out for sure that I am not going there.
Good luck JJ