Here's why:
* Collective nouns typically refer to groups of people or things (e.g., a flock of birds, a herd of cows, a committee of people).
* Peter, Phyllis, and Roberta are just three individual names. They don't form a natural group in literature or in general.
To create a collective noun for them, you could:
* Refer to them as a "trio" or a "group": This is a general and neutral option.
* Use a more descriptive noun: Depending on the context, you could call them a "team," "family," or "friends."
Ultimately, the best collective noun to use will depend on the specific situation and the relationship between these characters.