Make sure you have change of clothes/plastic bags/wipes to hand in the car in case of accidents of any variety. If 3yo is toilet trained, you might still want to take a potty with you as some toilet facilities at the roadside rest stops are, shall we say, a bit rural? 2 footrests and a hole - wouldn't fancy dangling a tot over those! (Fine if you go into service stations, but there are a lot of WC signs by the roadside picnic areas and some are old-fashioned)
Take the cleanest snacks you can - my kids tended to eat or sleep on the journey. Things you can easily pass back to them, not too fiddly. (And not bananas, you don't want to find the half eaten bit of banana that rolled under the seat)
If you are desperately brand loyal you may want to take your own nappies - personally I just took one pack to be getting along with then used a supermarket brand, I think Pampers/Huggies were v expensive.
Presumably you are past formula stage if you used it? Nestle is the big brand over there, didn't see any SMA (and can't remember what other brands).
Depending on what facilities you have in your holiday place (beach/pool/playgrounds etc) you can manage on far fewer toys than you think! Try to watch what they play with the most - things like a bag of Little People/Happy land and some cars maybe - not the sets that go with them, just the little characters.