Hey all, just to clarify things in this thread.
The values shown in the stat screen define the hit mass you can get through and not the number of targets. Each enemy has their own hit mass value, and hitting an enemy will subtract their hit mass value from the remaining hit mass “budget” for the attack - as soon as this reaches 0 or less, then the attack is stopped.
The Thunder Hammer, being a tank gestalt weapon, will allow you to hit a lot of targets but will only damage a few. So as mentioned above, damaging three targets is correct in this case, as the Thunder Hammer can only deal health damage to the first three targets it hits, but it can still keep going and knock further targets back.
Cleave and Power bonuses do increase the amount of hit mass you can get through, but do not change the damage settings for the weapon.
Cleave though shown seperately from Damage and Stagger will always be the same value.