1) The manufacturer of the charger should provide the needed information. If they are not forthcoming with the solar panel data required, maybe they are not so good with the solar panels themselves?
Maybe you found a great deal or have them already. So #1 wouldn't help.
2) Have you considered series/parallel wiring? This is where two cells are connected in series, and then it is parallel connected to the other two (which are also connected in series). Using the numbers you gave, you'd get 37.5 volts and 16 amps. The voltage and current should still be pretty good at non-peak solar conditions. You might still get say...20 volts and 10 amps early/late in the day. At peak, 37.5 volts should be plenty to charge a 12 volt battery!
Depending on the charger, parallel with all panels may be the most efficient. A charger should limit the current and drop the voltage to the battery, and a cheaper/poorer design may just use resistance to dissipate the excess voltage/current. I imagine the better charger could be much more efficient since they *might* have spent extra money on components and quality and used an efficient switching system.
Also, I believe the spec calling for 15 amps to charge is a maximum allowable current. Anything more than that can't be done or would cause damage to the battery. I never heard of a battery that can't be trickle charged - or charged with a lower current for a longer time period. In fact that is usually the most reliable way to charge a battery and preserve it's service life. Of course with a 150 AH battery, 15 amps is kind-a' like a trickle charge in that it is a relatively low current. Roughly, that would take more than 15 hours to get a full charge, possibly a lot more since the sun isn't often stationary at high noon.