[gruturo]

English is not my first language, but yes, to my knowledge that is grammatically correct.

Maybe it is a bit ambiguous, although from the context it should be reasonably clear that I was with my (now-ex) girlfriend for that period of time, and not with OkC itself :)

[freeAgent]

Most English speakers would say, "a woman who I was with for 3 years." In this case "with" is understood to mean in a romantic relationship.

I don't think what you said is technically correct. "With whom I was" calls for something (a verb or phrase) to be tacked onto the end, but it would still be awkward. For example, "with whom I was dating for three years."

[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.