3.0 L were only offered in an M3 in the e36 body style and in '95 only.
From '92 through '95, BMW offered the 318 and the 325, as stated before. '92 325's did not offer VANOS variable valve timing and thus are significantly slower than their '93-'95 brethren. A 3.0L engine was offered in late '94 (sold as model year '95) and '95 in the M3.
From '96-'99, BMW changed their offerings to the 323 and the 328 due to OBD2 federal standards. The 323 had a 2.5L engine that is slower than the '93-'95 version. The 328, as stated, offered a 2.8L engine, adding 1hp and 25ft-lbs of torque to the previous 325. The M3 engine displacement was bumped up to 3.2L, increasing the low end torque, but not adding any hp.
As for which car to purchase, that's up to you, and entirely depends on what you plan on doing with it. Since you're asking this question, then my bet is that a '92-'95 318 would work out fabulously for you. I'd stay away from the 318ti, however, because, well, it lacks an independent rear suspension and I'm just biased enough to say that it's my least favorite e36 offered.