it'a all confusing to me, about what lines and tickets i need. If I want to get from Louvre to where I stay (Résidence Courcelle) is it okay to walk to the Opera and take the line 3 Metro? Do I need only one ticket for that? It looks that where I want to go is outside the T1 and T2 zones.
Paris Metro is all one zone requiring only 1 Paris Metro ticket. With RER trains, zones are applicable, but not with Metro. Paris Metro is all one zone, even if the Metro station is in say RER Zone 2 or even Zone 3.
You can use Paris Metro line 3 with a single Metro ticket to go from Opera to Louise Michel station, which is closest to Résidence Courcelle.