When this term first start being tossed around regularly I was adamant that it was being improperly used. Trust me, I am well aware that it is the standard now for about a billion people. Back when it was first being uttered more often than "ship it" I couldn't convince mostly anyone that I was, in fact, correct.
Here's my contention. In most NLHE games, you have a SB and a BB. Sometimes you have an Ante. Let's leave confusing additions like straddles off the table for now to simplify. That BB is a "forced contribution" to the initial pot. Don't believe me? Get up during a tournament when you're about to be the BB and watch them take it off your stack without consulting you. Not exactly a choice. Therefore, it is NOT a bet. Limpers are also NOT making a bet. They are calling. Only openers, those that "raise" anything min or above are actually making a bet. The FIRST bet. Therefore, when the next player raises even more, he is actually making the SECOND bet, NOT the THIRD. Therefore, it is NOT a 3 bet.
I'd be shocked beyond belief if ANY of you agree with me on this. It is basically gospel and canon at this point.
Hell, I'll be shocked if the stats on this thread go more than 10 views and even 1 reply, even if it someone calling me a complete moron who is totally wrong.