The R3 hotfix primarily focuses on cross-platform compatibility and system stability for Mac users:
While highly stable, 10.3.10 is strictly bound to specific environments. It is officially qualified Avid Pro Tools HD 10.3.10 R3 hotfix only For Mac
Improves overall performance on Intel-based Macs running OS X 10.6.7 and higher. System Requirements & Compatibility Avid Pro Tools HD 10.3.10 R3 hotfix only For Mac
This hotfix addresses known stability and compatibility issues for Avid Pro Tools HD 10.3.10 R3 on macOS. Apply this update only if you are running Pro Tools HD 10.3.10 R3; installation on other versions is not supported. Avid Pro Tools HD 10.3.10 R3 hotfix only For Mac
Why go through the trouble? If you are on Pro Tools HD 10.3.10 (base) and experiencing the following, the R3 hotfix is mandatory:
Avid’s hotfix naming convention: