New to the game and trying to find a good senario to play. Also if I have a unit within hand to hand combat range og 1 or more enemies, do I have to att them or can I shoot at another unit that is farther away? Why is hand to hand combat 3'' away and not base contact?

There is no state of "in melee", so you can shoot no matter how close you are to another unit. I have no idea why 3" for close combat (although it seems like a good distance). The reason for not base to base is that all miniatures must remain 1" apart making base to base impossible.

Mike J is correct.

Close Combat is not a game state, it is an Attack Action.

The only time you cannot shoot at an enemy is if they are attacking you with a Close Combat attack and you make a Reaction Attack.