Hi OP, I really want to help you but have not got much time, apologies for the copy and paste, my DD is 2 years 10 months and I'm answering as for her.
So what would you expect from your 3 year old when you're out.
DC 2.10
Would they be in the pram? No - we have the buggy just in case we are going on a walk of say 45 mins plus and she may resort to the buggy.
Would they need reins? No
Can they tell you if they need a nappy change or if they're hungry? Will say if she need the toilet [she trained at 2 years 5 months] Will ask for food regularlythrough the day despite obviously providing her with three full meals plus snacks of fruit!
Do they need nappies? No, but does still need a pull-up at night.
How often do you need to stop on a 5 minute walk if they're not a pram? (And walking alongside you) She would not need to stop.
Do they travel on public transport very well? Yes, this is not something I would have to think about prior to leaving the house.
Could you tell them not to open the door on a taxi when it's in motion? Yes - she wouldn't do it, so I wouldn't have to tell her, it's a given.
Could you take them food shopping? Yes, I took her earlier today, I asked her if she wanted to get herself anything and she said various fruit and then she said sausage rolls (random), so we got those and she was happy.
For your DS, always put how it affects him WHEN HE IS AT HIS WORST, not just on a normal / good day. I don't mean to be bossy or patronising, just genuinely want the best for your DS. Good luck.