Cases are considered mild by default. The 3% serious is just that not everybody is reporting the breakdown. Another part of the calculation is that since many of the cases are identified via contact tracing - not everywhere, but some places - they are still pre symptomatic (Rand Paul, for example). And some are symptomatic, but not serious *yet*. This seems to come in 2 phases for many victims.
From a doctor I know that is in the middle of this mess, the number is 80% mild, 20% serious or critical.
That is what a dr we know in Houston is saying as well.