A 3/4 system consists of 4 bets which transpire from 4 picks -- four 3-pick multis (parlays). At least three of the four picks must be correct to gain some winnings - the exact amount of the winnings depends on how many of the predictions prove correct.
The term 3-bet most commonly refers to the first re-raise before the flop, although the term also refers to the first re-raise after the flop. (This article focuses exclusively on preflop 3-betting.) Subsequent raises are referred to as a 4-bet, 5-bet, etc.