Well, you are right about a peak charger not being able to do a "formation" charge because it is a 15 hr and forced charge. The peak needs to be ignored.

The charge will last 1 hr or so if the discharge rate is at 3 amps, same as the charge rate being at 3 amps will last for an hr.

It is a rate, like a 3 gallon bucket being filled or emptied at a rate of 3 gals/hr