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.
For example, you have selected five wagers, you may choose to bet on the system 4/5 . This means that you will receive your winnings if at least four of your five wagers are correct. The same principle applies if you choose a system 3/5 ; you will receive your winnings if at least three of your wagers are correct.