Italy doesn't infringe on its obligation to keep its deficit below 3 %, but on its obligation to keep its debt-to-GDP ratio below 60 %. Most countries in the European Monetary Union (EMU) have an excessively high debt level after the economic crisis of 2007, but all countries seem to be engaged at stopping the debt from spiraling out of control.
Italy is the outlier. It openly defies the rules of the EMU. The Italian government doesn't seem to be serious about stabilising or even lowering its extremely high debt-to-GDP ratio (about 130 %). The government wants to simultaneously increase its social spending and lower its taxes. Higher spending and lesser income doesn't lower one's deficit. Sure, the Italian government predicts that its reforms would unleash economic growth, but neither the markets nor the non-Italian EU politicians believe in these predictions.
It's not only about Italy being a bad example. (Others have been, too.) The Italian budget directly threatens the economic stability of Italy and the survival of the EMU. If a country with a lower debt-to-GDP ratio (e.g., Estonia, Bulgaria, even Germany) did so, nobody would be seriously concerned - but in the case of Italy a default seems imaginable. And Italy is not one of the smaller member states! The European Central Bank is already owning a huge pile of Italian government bonds and would be obliged to continue buying them in large quantity. The markets don't want them as shown by the large spread. (= risk premium, the difference in interest between Italian and German government bonds, which are considered almost risk-free) The ECB-owned bonds would lose much of their value. Furthermore, the citizens of the other Euro countries would be on the hook for an Italian banking crisis.