You mentioned you had bought the T3 heavy… Only one tier will count towards the achievement one set of t1 one of t2 one of t3… you can buy one piece light a different piece medium if is the same tier it will count but it you had already bought lets say the Heavy helmet of tier 1 and you go back and want the Light head piece of T1 too this second one will not count as you had already gotten a Helm from That tier before.
Hope i explained myself sorry if I confused you.