Bluetooth problem SOLVED!
I did an online chat with Google support. They were helpful and professional but it did not solve the problem with my Pixel 3 not connecting automatically to my car Bluetooth. After the chat, I discovered that I could get it to connect by going into the car Bluetooth setup "Select Phone" and select my Pixel 3 and it would then connect immediately. (I had been going into the Previously Connected Devices in the phone settings and connecting from there.) However, after selecting the phone in the car setup, it didn't seem to remember it once I turned off the car and came back. I had to select it each time. This made me suspect the problem was with the car, not the phone. So, I Googled for this problem for my car (which is a 5 year old Subaru) and found the solution. Turns out you have to use the hands-free voice setup to pair the phone to the car Bluetooth, rather than go through the Bluetooth setup using the radio controls. The instructions in the car manual are terrible and give no hint that the two methods of setup behave differently, but I did the pairing using voice commands and it seems to be working.
I figured I'd post the solution here in case it is helpful to someone else who may run into the same issue.