You need to complete 5 matches as each variation of Triborg: Sektor, Smoke, Cyrax, and Sub-Zero.
To select Sub-Zero, you need to tap UP twice, then select with 'Y' (gamepad) 'I' (keyboard).
You unlock items in the Kombat Kard menu after 5 wins, so you may check there to see which ones you have yet to do.
You can combine this with grinding the "Kustom Kombat - Brutality" method of obtaining Koins/FXP/XP as seen in other solutions.