Try using a program called ClassicTrigEdit, which can be downloaded by itself or with StarDraft from
www.stormcoast-fortress.net. It's a nice tool for streamlining trigger-making.
As for that trigger, assuming you meant every five kills garners three minerals and you're using ClassicTrigEdit, it would go as thus, I believe:
CONDITIONS:
Set Score: Current Player kills score is exactly 5.
ACTIONS:
Set Resources: Modify resources for Current Player: Add 3 ore.