もう3ヶ月洗濯していません。

x から means since x. E.g. 一ヶ月から = since one month. So examples: 6時間から、まだ食べません。 Rough translation: Since 6 hours, I haven't eaten yet.

You could say: "I haven't done my laundry in 3 months/ for about 3 months already" I don't think you need to add the word "already" to that sentence unless you want strongly express that you haven't done your laundry in three months. This is only my opinion so I could be wrong, either way I hope I helped in answering your question.

I imagine it is: 3月から、洗濯しません。 Translation: Since three months ago, I don't do laundry.