請問"私", "僕"同"俺"用時有咩分別?
係咪"俺"一定要男人先用得?
"僕"就男孩用?
但有時好似見有男人用"私"....

Men can use all 3,
俺 = usually used when with friends, family. Most common form of "me" among men.
僕 = usually used by young boy. Also preferred when talking with those older than you/ above you.
私 = Almost must be used in formal situation, can be used anytime actually.

Depends on preference.

我唔可以話識

私比僕同俺有禮貌
我未聽過有女仔用俺
僕記得女仔好似唔會用到
男人用私都好正常

以上只係小弟學到依家所識
有日文高手在場請指點錯處

女仔5用得俺/僕
静係用得私/あたし,旦係好粗魯果D有機會用俺