BleepingComputer.com: Other than MBAM and AVG free

You shouldn't have AVG and Avast active at the same time, if they both have active scanning, there will be a conflict.

Using multiple antivirus and firewall applications simultaneously can cause problems due to software conflicts. And performance will be greatly impacted.

Of the two Avast or AVG I would choose Avast. AVG is good to, that was my choice for many years, but Avast is better today. Threatfire in tandom with other antivirus programs don't often conflict. That's why I run it at the same time as Avast. I also scan with SAS often too.