i think both aamc 3 and aamc 5 are good indicators of your real score, but i'd take aamc 5 since it is more recent.
i found it to be a bit harder than aamc 3 as well. just out of curiosity, how did your scores compare on aamc 4 and 6?
i've taken 4 already and am planning on taking 6 tomorrow.
i scored 2 higher on aamc VI. aamc VI is my highest score so far (but is also my most recent & i've done a lot of physical sciences studying in the last couple of weeks). they seemed like pretty comparable tests to me.