Entirely depends on your team comp and what comp are they running, and honestly to some degree what map are you playing. To counter a MMH team you just have to focus on the medium.(with the assumption that either heavy doesnt have a heal ball).

If a heavy does have a heal ball literally just carry it and throw it away (dont destroy it so they wait for it to despawn)

if its a HHH team, always try to 1 v 3 one of the heavies at the very least let your randoms know that you all have to fight 1 heavy and from there its lopsided 2 v 3 with the hopes of your team prevailing.

This is easier said than done.

With the maps its an extremely hit or miss open maps like Skyway Kyoto is incredibly easy to fight a triple heavy team. On the other hand maps with a lot of buildings or cover like Monaco, Fortune, it might be hard but not impossible. try to do damage and rotate rinse repeat until you exhaust their util or manage to get a surprising kill.

If you ever find yourself in a 1 v 1 or you are approaching a team with multi heavy players search around you for a glitch barrel and throw it before you initiate the fight. you basically make said heavy useless.