I think its a good idea to think about how long you re likely to be out for and if you re more likely to be driving or walking, buggy or carrier/ wrap etc. obviously there will be a bit of everything.
I, like you, on the 3rd child ( albeit different ages), spent a lot of time nipping back and forth in a car; school, swimming lessons, dance lessons, supermarket etc
I have hardly walked anywhere ( dd2 was at nursery though every morning, so my day was chopped into bits!).
I kept one nappy in the change bag and a couple in a separate bag in the car so that I could refill bag if needed
I ve got a portable change mat that has a few pockets so wipes, hand santiser, spare nippa went in there. Then I can move things from bag to bag and hubby can put in his more manly messenger type bag.
I d definitely recommend a bag you can put on your shoulder or on your back, or cross body etc. you need your hands