Okay, but why I can't I use my off-hand attacks for it then? and why a rogue (Thief) have 4 attacks on level 3?
Even though you have two weapons, when you use an action to attack you should use the one in the main hand, then you can chose if you want to use a bonus action for the off-hand attack. Both should be able to inflict sneak attack, AND, you shouldn't be able to use an off-hand attack if you didn't also used an attack action at the same turn. Otherwise it means that characters with off-hand get to attack with a bonus action.