It's already been three years since we got our cat.
We have already had our cat for three years.
この場合、次のような言い方ができますよ。
ーIt's already been three years since we got our cat.
「この子がうちに来てから早3年だよ」
ーWe have already had our cat for three years.
「この子をもう3年も飼ってるんだね」
ご参考まで!