After washing my hair with just hot water for three weeks, my hair isn't oily anymore.
I didn't use any shampoo, only hot water for three weeks, and now my hair doesn't feel greasy at all.
ーAfter washing my hair with just hot water for three weeks, my hair isn't oily anymore.
「３週間お湯だけで髪を洗うようになって、髪がベタベタしなくなった」
to wash one's hair で「洗髪する・髪を洗う」
for three weeks で「３週間」
oily で「脂っこい・脂性の」
ーI didn't use any shampoo, only hot water for the last three weeks, and now my hair doesn't feel greasy at all.
「ここ３週間、お湯だけでシャンプーを使わなかったら、髪が全くベトベトしなくなった」
for the last three weeks で「過去３週間」
to feel greasy で「べとつきを感じる・脂ぎった感じがする」
at all は否定文で「少しも〜ない」
ご参考まで！
After three weeks of washing my hair with only water, it stopped being greasy.
"washing my hair with only water" は「お湯だけで髪を洗う」という意味です。"stopped being greasy" で「べたつかなくなった」という状態の変化を伝えています。
お湯だけの洗髪をはじめて、最初のころはべたついたが、その後髪がきれいになりました：
"When I first started washing my hair with only water, it was greasy, but after a few weeks, my hair became clean."
カナダ 
アメリカ合衆国 
