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For shotgun the main thing to practice first is reloading. New shooters lose a huge amount of time here. Get some dummy shells and at home practice loading them, preferably while moving. If you want live fire at the plate rack try shooting 2 plates, reload 4, shoot 2, etc. Then there are slugs. Here you're treating the shotgun a lot like the rifle so the first thing is to check where your slugs are zeroed at various ranges. I've seen slug shots at matches from 2-100 yards. It depends on what match you're going to. Then also practice pulling the trigger smoothly as it is a bit heavier on the shotgun than a rifle.

For rifle practice much of the movement and set up for the close range stages is very similar to USPSA. I'd practice transitions keeping both eyes open and getting your eyes to the next target before you snap your gun there just like you would the pistol. Move between positions and get the rifle up quickly as you arrive, also practice shooting on the move, the mechanics of which are similar to pistol on the move. Long range rifle know your zeros out as far as you need to for your next match. If you're just shooting locally that may be 100 yards but major matches that might be 500+ yards. Practice shooting offhand, kneeling, prone, and from barricades of all types. Try getting as many points of steady contact you can to get a stable position. You will shoot from all kinds of improvised positions in 3 gun so try out as many as you can and figure out how to best set yourself up.

There's dozens of things you can practice for 3 gun. It gets a lot more complicated than pistol only. Those are some places to start. Once you go to your first match identify what areas you need to work on and figure out drills and dry fire practice to get better at them.