The '3 in 1' option should be used in conjunction with '3 in 1' detergent tablets or gel packs which combine a detergent, rinse aid and drying agent into a single packet. The '3 in 1' option will automatically adjust the cycle settings and sensors for modified cycle times.
3 in 1 tablets or gel packs are often used when the water source is "hard" or contains high levels of minerals that cause residue buildup on dishes.
UPDATE: The '3 in 1' function is no longer necessary as an option as the built in wash cycle sensors no longer require the manual selection of the 3 in 1 option. This feature / button option will be removed from future productions of EdgeStar countertop dishwashers.