"Je to uz tri roky zenaty" does not mean anything (or in Common Czech it would mean "It is has been married for 3 years already").
"It is already three years of marriage" would be perhaps "Jsou to už tři roky našeho manželství."
Manželství is the official partnership that is entered by marriage (svatbou). Ženatý is an adjective describing the marital status of a male person.
Cannot comment on your other question with is vs. has been.