The smallest is a small fighter which was refitted as drone. It is build against other drones and small fighters.
The middle one is a destroyer class which is at least in my story line the newest of all. It was build to replace it's old aged predecessor and have a better punch on the middle class pirate ships.
Because a commando ship was missing, someone decided to build one out of the remaining hulls from scrapped base towers. The Gun was removed. The energie of the turning axis was rerouted to create extra thrust. It's main purpose is the oversight of battle. Also fitted with a jump inhibitor it also takes away the priates greatest weapon: Run.
All of them are designed as patrols to safe the borders from pirates and other unwelcome guests.