question about temporary tablespace group

Hi Everyone,
If I have a tablespace group consisting of three temporary tablespaces, and if a user default tablespace is one of the temporary tablespace, instead of the group, will the use able to use the other temp tablespaces in the group if its default temp tablespace is used up?

If I change all user default temp tablespace to the group, will that eleminated the need to balance the space for the three tablespaces until all of them used up?

Thanks.
Yinhai