1-2-3 is short hand for describing the standard gcd combo system used by most classes and jobs of the game. 1 is the action that is uneffected by the action performed before it, 2 is the action that get its combo bonus from following action 1 and 3 is a action that gets its combo bonus for following 2. A majority of the non-caster classes and jobs (along with Red Mage) use variations of this system to perform their weaponskill combos.
For Gunbreaker the basic single target combo is:
- Keen Edge
- Brutal Shell
- Solid Barrel
While the Cartridge/Continuation combo is:
- Jugular Rip
- Abdomen Tear
- Eye Gouge
Not really. The combos would still be Keen Edge -> Brutal Shell -> Solid Barrel and Jugular Rip -> Abdomen Tear -> Eye Gouge.
Are you certain you are including all your abilities (including role actions) on your hotbars? Gunbreaker has 32 buttons (25 job abilities and 7 role actions) at level 80 not including general actions such as mount, sprint, teleport and limit break. The basic controller Hotbar set has 16 buttons while the expanded can hold up to 32 without changing to another hotbar set.