You switch between fire modus by going into Gunsmith before a mission starts and then change the trigger mechanism.
When you selected 1/2/3 or 4 enemies you need to wait untill all of them are blue
(''notice that if you select 4 enemies you will need to target one of the enemies yourself, you can do this by zooming in and keeping your aim on the target untill you see a laser target from your weapon to your enemy or untill the number of the one you have targetted also gets blue'')
When all of the numbers on the top middle of your screen are blue you can hold the RB button and they will shoot the targets they aim on. it also works when you target one of the selected enemies, your team will shoot as soon as you shoot.
Hope this helps if you didn't find it out yet