三个月前 sounds more native than 三月前, cause 三月前 May cause misunderstanding that “before the March” : )

三月前 means before March 三个月前 Menas three month ago But if you said 三周前 三天前 三年前 all means “ago”

I don't know if it's exactly the same in Mandarin, but in Cantonese, you have to put a classifier (个) whenever you talk about months or weeks (三个月, 三个星期). You don't need a classifier for days or years (三日, 三年).