Final answer:
The prevalence of the three variants of the AVPR1a receptor in humans is not specified in the available information.
Step-by-step explanation:
According to the available information, it has been reported that there are three variants of the AVPR1a receptor in humans. However, it is not specified whether every human contains all three variants or if some individuals may have fewer than the three variants found in the total population. More research is needed to determine the exact prevalence of each variant among individuals.