event to validate datagridview rows

Question

 i am not able to understand which event should i use so that i can check if 3 of the required fields out of the 10 fields are filled by the user.if the 3 required fields are filled in , then only i can let user go to enter new row.

 

this event should fire each time before we go to next row. 

 

i am pretty new to c#. thanks for the help