clydesdale,
I'm no electrical engineer but here's my understanding of batteries and capacity:
A 3 ah (amp-hour) battery will theoretically put out, for example, 1 amp continuously for three hours. The (number of amps being consumed) x (number of hours running) = amp-hours.
If I understand you correctly one LED light fixture consumes 3 watts. So running two would use 6 watts. The formula for watts (learned in 7th grade electrical shop class) is Amps x Volts = Watts. So doing some algebra, Watts divided by Volts = Amps being consumed.
If your LED's run on a 12 volt battery you get 6 watts divided by 12 volts = 1/2 amp. A 3 ah battery would theoretically run 1/2 amp for up to 6 hours, but in reality I don't think you can practically get that much use out the the battery to have it run with adequate power. So I would guess it might practically run for 2/3 of the theoretical time which would be for 4 hours
What you would need to know is how many volts do the LED's take when running. You need to know this anyway to get an SLA (sealed lead acid) battery of the right voltage. I hope my thinking has been helpful.
David