Can I equip the same weapon multiple times? So 3 88 canons?

I dont think so, i believe a unit can ever only have one of each upgrade card. Somewhere in the RR it says so though i cant say where right now, this question was raised in another thread recently.

Page 5 of the rules reference, last sentence of the first paragraph under the "Upgrade Cards" header:

Quote

A unit cannot equip more than one copy of the same upgrade card.