Is Catholicism a polytheistic religion?

In the most technical sense, no, Catholics are not polytheists. The idea of the Holy Trinity is not exactly that it is a "team" of three entities, but that there are three forms that the same God takes. Since Christians believe that God is omnipresent (all places at all times) and omnipotent (all powerful), these "forms" of God are ways of thinking about God rather than separate beings. Even Jesus, the Son, is still simply a manifestation of God on earth, not a separate being from God. As for the saints, Catholics do not pray to them as gods themselves, but as representatives who are able to help get the prayer to God more swiftly and surely.