Backrent after moved out

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Need help with backrent!!!

3 of my tenants just moved out after a year's lease and returned the house. They currently owe me $6000. One of them wants to pay back his portion of the balance. (the lease doesn't specify whose portion is what, they are in it together). If the other two decide not to pay. What should I do? If I turn it over to collections. Would the one that paid his portion be affected?  Does anyone have any experience working with the collection? how do they handle the backrent situation?