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My question is that if I'm facing a tau army and they have the ability to give my units gets hot within a certain range. If my units are noise marines with sonic blasters do they roll once before the shooting phase to see if their guns get hot, or is it every time they fire (up to three times per model)?

If a model rolls to hit, each shot ha a chance of overheating.
If model shoots a weapon that doesn't roll to hit (blast templates for example) then you roll before shooting.

If a weapon has gets hot it works exactly as the rule gets hot says it does.
If you know plasma guns, sonic blasters would work the same.