I was a landlord for 20 years. Let me give you the landlord's point of view.
Three people live in my rental. Three people signed the lease. Three people pooled their money and gave one deposit (despite possibly having written 3 checks).
One person moves out at lease expiration.
The other two qualify for and sign a new lease.
The deposit remains the same so I would be under no obligation to refund any of it to just one tenant, I would leave that up to the tenants to work it out among themselves.
In other words, you get your 1/3 back from your roommates, not from me.
Or, your roommates come up with another 1/3 for me and then I give you back your 1/3.
Either way I still have 100% of the security deposit and, frankly, I wouldn't much care how that result is achieved.

