As a mentor, all of them are eligible to attend the summit since they're already familiar with Debian to represent there.
The mentoring cannot be quantitatively measured and compared.I would suggest to go for simple queue (first come first serve) rather
than the complex algorithms.
We could also split the fund for three if one of them is traveling from nearby country. So, I would suggest the interested people to
include their names in the waiting list. Others might get the chance next time as far as they continue to be a mentor.