How long do I Charge my mower battery with 3 amp charger
If you take the figure stated on the battery (ie 10AH) and divide it by the 3 (3A of the charger) then that will give you a reasonable time in hours.
But it does depend on how discharged the battery is initially. A battery that is dad flat will take a while to start charging and take longer overall, if that is, it will recharge.
A battery that is half charged will need less time.
However, leaving a charger on for slightly too long will only cause some gassing as long as you don't leave it for days.
If you have a timer (one of those cheap light timers) then set it for 2/3 of the time you calculate and see how the battery comes up.