Mine are now 3yo, 2yo and 4 1/2 months but I have always done what you just said. I park next to the cart return and pray there is a big one out there. If there isn't a big car cart then I put my 2yr in a regular cart, baby in the carrier in the cart then my 3yr walks into the store. Once I get into the store I switch to the double car cart. It's a lot of work getting in/out of the car but once you do it a few times it starts to get faster.
I go grocery shopping on thursdays when I have all 3. Now in the store and keeping the big boys from touching each other in the cart is a whole other issue. I really want to build a divider for that darn car cart so I can separate them sometimes.