How hard is it to use a PEC inverter in the Philippines?

Not so hard for Philippines as we are always Hot. PEC way is to use 6 percent. Proper way is to get heat coefficient data x 15 degrees ( Temperature adder) for max voltage VOC. GW 3000 on 12 vs 14 panels. Too little it will shutdown during middle of summer ! Minimum inverter start up voltage to minimum panel size. Use Vmp plus a heat derate.

Which type of inverter is best for a solar system?

You can use single phase inverter to correct phase imbalance in a system. Technique of choice if MDP or distribution panel is already overloaded. Hybrid Off Grid Design. SOLAR, then Battery then Utility. Cons: Solar does nothing after batteries are charged and loads are satisfied. Highest form of offgrid system.

Which Inverter should I use?

Inverter 1 to line AB, inverter 2 to line BC and inverter 3 to line AC. You can use single phase inverter to correct phase imbalance in a system. Technique of choice if MDP or distribution panel is already overloaded. Hybrid Off Grid Design. SOLAR, then Battery then Utility.