That is so sad, Gissele
Tara - could you have Graci walk alongside of you, the newborn in the carrier, and Ava in the front basket? Or for small trips, put the two smallest in a double stroller, have Graci walk, and put the items in the basket underneath the stroller. That is what I do with the twins now. When they were infants, I would only do groceries when dh or another adult was there to help. It got stressful if they both needed to nurse while out in public.
Back to the topic, I totally agree with what you did, crisi, and would have been disgusted by the reaction of the mom and police officer. I always park right next to a cart return so that I am never more than a few feet away from my kids at all times.
I have parked in our driveway with the car locked and the twins inside buckled while I ran into the house to grab something, but never for more than a minute. We also are on the end of a court, and three of our neighbors are retired and often out and about.