In level 3 cars, the driver does not need to pay any attention until it is asked to do so by the car, and is given some time to respond. From the spec:
At level 3, an ADS is capable of continuing to perform the DDT for at least several seconds after providing the fallback-ready user with a request to intervene. The DDT fallback-ready user is then expected to achieve a minimal risk condition if s/he determines it to be necessary.
Waymo cars require constant attention by a safety driver or two.
These are completely different situations.
Edit: the human backup the Waymo needs is not the human backup the spec requires for a car to be defined as level 3.
Edit 2: ADS is Automated Driving System, and DDT is Dynamic Driving Task (driving).