Transact SQL :: Generate Unique Username From Data Columns

I am trying to achieve the below problem statement, however I am getting stuck at the looping part.

EX: We have three columns

First Name |  Last Name | Mothers Name

I want to generate the username field using above columns, so lets consider, I have following data

First Name |  Last Name | Mothers Name
a                    b                  cdef
a                    b                  cdfg
a                    b                  cdfj

Expected Usernames:

1: a.b
2: a.b.c
3. a.b.cd

Basically, it has to be FirstName.LastName.(incremental letters from MothersName until the name becomes unique)