I wear at least three layers, an undershirt, a shirt, and something thicker on top.
この場合、次のような言い方ができますよ。
ーI wear at least three layers, an undershirt, a shirt, and something thicker on top.
「肌着、シャツ、何か厚めの上着、最低3枚は着ます」
at least「少なくとも」
three layers「三層・三枚重ね」
an undershirt「肌着」
something thicker on top「上に何か厚めのもの」=「厚めの上着」
ご参考まで!