It means Half your bet is on 3 and half is on 3.5. So for instance if you bett 100 on the over and it’s -110 you really bet 55/50 on over 3 and 55/50 on over 3.5. If the total is under 3 you lose both, over 3 you win both. If it lands on 3 you push on your over 3 bet and lose 55 on your over 3.5 bet