How to kill a background running session in dialog programming (SE80).

I am new to ABAP. I created a dialog program with 3 transactions (say a,b,c). After going into transaction 'a', I am calling other two (b ,c) transactions(using call transaction functionality). But, whenever I call a new transaction, earlier transactions is running in the background and after a few attempts maximum session limit is exceeding and the programming is terminations.

Please let me know how to kill the background sessions dynamically after entering a new transaction.

Also, I am having a push button for leaving the program of type 'E'. Due to these numerous sessions, the EXIT functionality is taking me to the earlier session, but not directly to the program. Kindly help me here also.