You don't say the current ages of your kids - your profile says 4, 2 and 9 mo.
NO WAY.
I would run errands when someone else could watch them, I'd pay a babysitter for a couple hours, trade out care time with a friend or I would only go places that I could bring them with me.
My daughter is 11 1/2 and I have only let her "sit in the car" for the past year or so.... and THAT's only if it's literally going to be minutes. Although..... last week I did run into Babies R Us to buy a pair of baby shoes for my niece. So, my daughter stayed in the car for about 10 minutes. But that's a decision I made based on the area of the store, where I was parked, time of day and temp outside.
I'm not worried about abduction, necessarily.
What if the little one spit up and vomited? I wouldn't want her to be without air for a minute. What if one of them unbuckled their seatbelt and climbed over the seat and knocked the car into neutral and it started to roll?
So, NO. I know it's tough to live in an area with no family and friends, but no way would I do this and Yes, I would call the police if I saw kids that young in the car alone.
Just my $0.02
ETA - typically kids can stay alone around 11 or 12. Most states have "guidelines" of things the police would investigate - like can they call for help, do they know what to do in case of an emergency etc. Again, my daughter is 11 1/2 and we are JUST NOW doing this. Your kids are WAY TOO YOUNG.
I know you don't have help or support. It's up to you to FIND SOME. make friends, join a mommy and me class, get your kids registered for mothers day out or whatever to give yourself the time to do your "work".