3 roommates, 1 moving out, re-newing lease - dealing with deposit

[Peter Hroph]

Appreciate in advance for advice. 

I plan to re-new lease but unsure how to deal with deposit and last month's rent. I have 3 great tenants who are roommates - who also paid last month's and deposit in '15. One of them is moving out, and 2 remaining will be staying. I only get one check from them on monthly basis, they divide amount between each other, equally.

Oct 1 is the date when lease expires. Instead of 'returning' deposit from '15, and having two remaining write new check, can anyone suggest wording/letter that I could have the moving out tenant sign stating something along the lines of 'I, ____, have received my full deposit back, including interest, from my roommates Xname and Zname and do not expect any additional monetary return from my landlord, _____' ? 

Should I deal with it differently? They are nice young working professionals, and I have no doubt they will, if not already, return their deposit share back to the leaving tenant. However, I would like to protect myself.