What happens when a eurozone country breaks EU fiscal rules that limit the allowable deficit to 3% of GDP?
Most states have at one time or another broken the limit. France hasn't met the limit since 2007. There has not been a single measure taken by the EU.
The deficit "limit" of 60% is also merely theory, with the EU on average being at 90%. This is of course less realistic to enforce on short notice, but the EU does not even enforce the "60% or decreasing" rule.
Fun fact: There has not been a single member that complied with all the requirements of joining the EU when they joined. But as long as the countries attempt to make progress and are not problematic, the EU doesn't intervene.