Quick answer is that Patrol 1 covers all county and state units. (i.e Deputy Sheriff's, Texas Highway Patrol, TABC, TWPD (Game Warden and Park Rangers), DA Investigators, etc)
Patrol 2 covers one half and Patrol 3 covers the other. Patrol 2 and 3 are for the small cities that DCSO dispatches and the constables. It should be noted that Patrol 3 is not always in use as during slower nights they either patch the channels together or move everyone to Patrol 2.