NEMA 10-30 Charging for the first time at home. | Tesla ...
According to Tesla's Mobile Connector adapter's page, a NEMA 10-30 adapter should add 22 miles of range per hour on a Model 3, so it sounds like you're getting exactly what you should. You can also check your display for the kW charge rate, which should be ~5.76 kW (or 24A x 240v).