[gruturo]

> Most English speakers would say, "a woman who I was with for 3 years."

Actually, I'm pretty certain you're supposed to use "whom" there, not "who".

[WorldMaker]

From another perspective: whom is one of the few relics of the entirely dropped dative case in English (via the also disappearing objective or oblique case which mostly only remains in English personal pronouns I versus me). It's generally relegated only to formal writing, and some (including the Oxford English Dictionary since the 1980s) consider "whom" to be entirely deceased, ancient, and no longer an active English word.

[freeAgent]

It depends on whether you are using who/whom as the subject or object. Who = subject, whom = object.