what is the equivalent incandescent wattage that is equal to a 40w halogen G9 type T4 bulb? Is a chandelier with 3 40w G9 bulbs enough to light up a 12 x 12 room?

40 Watts of incandescent is the same as 40 watts of halogen. A halogen lamp is an incandescent lamp ,with a special glass to prevent the tungsten filament from evaporating too quickly. But most halogen lamps are clear glass and a very bright point source. The regular light bulb have a more dispersed point of light and usually a coated lamp.

So, the "quality" of light is very different between the two.

If you have a fixture with the G9 base, why asking about an incandescent equivalent ? You can only use a G9 lamp in it.

120 watts of light from either halogen or incandescent seems a bit low for a 12x12 room, but depends on many variables, type of lamp, type of fixture, color of walls and floors, and ceiling for that matter, age of occupants, task in that room.

It may be just the ambiance your looking for, or someone else may think it is dim. Residential lighting is always very subjective.

Rooms like dining rooms are better overlit a bit, then use a dimmer for those times you want less light.

Explain more and we can perhaps help better.