If you noticed your daughter bump into the woman and didn't say sorry and talk to your daughter about it, then I can't blame the woman's husband for being upset. They were expecting an immediate apology from you and a verbal lesson/redirection from you to your daughter. They felt like you were telling them they were being unreasonable when you said "she's only 3".
We used to split up too, mostly in restaurants. I would take the babies outside when they cried, but if they were too upset, my husband would get the food to go instead and we would leave.
There is a metal bar between the grocery lines at our favorite grocery store. My daughter starts hanging on the bars the moment I start to unload the cart. Every time, I have to tell her to stop. I still have to let my daughter know not to treat the grocery store like a playground, even though it is too tempting for her to take my advice just yet.
I once saw a toddler teetering on her butt on the back ledge of a high chair at a restaurant with tile floors. I couldn't stand it any longer and I went up to the family and told one of them quietly that the child looked like she was going to fall. I could tell they felt judged... They sat her back down and whispered to each other...someone then glanced over at our table and didn't say a word. If a stranger called my attention to something like this, I would probably be a bit embarrassed, but I would also say "THANK YOU!"
I live in an area that is hot and humid almost all year round. I was in the grocery store once and my daughter didn't have her flip flops on (they always fell off when she was in the cart, so I would take them off once I sat her in there). An "older" woman said it was stupid of me to not have shoes on her feet. I've had older folks tell me my kids weren't dressed warm enough either! I laugh now, but I know I was pretty ticked at the time!