As far as I understand it. The first chips put into a pot in each round of betting is called a bet. Therefore pre flop this is the BB. Post flop the first money put in is called a bet. Therefore the 2nd amount put in is a raise, so either an open raise preflop or the 2nd bet after the flop.
So therefore a 3 bet is either the 2nd raise pre, or 3rd bet post. This makes sense, if you just count the number of bets/rasies. A 3 bet should always theoretically be the 3rd bet/raise.
I had to google this, as I'd fallen foul of not saying raise in certain situations, and I hadn't quite worked out why, so I just started saying raise in all situations. One total knob decided to pull me up on it, without actually taking the care to explain why. You get these arrogant posturing idiots live. Anyway, later I should have doubled up through him, as I again said raise at him with AQo I think, he called with AJo, Unfortunately he spiked his J, so I couldn't get my revenge.