1. The side wall nearest the speaker 2. The rear wall 3. The side wall across from the speaker
A secondary factor is the speaker-to-speaker time constant.
When you use this Golden Ratio method to set your room up, the speakers are placed so the three nodes progress or differ from one another in Golden Ratio. This eliminates any unison or near unison resonance in the nodes.
Panel or dipole speakers such as Apogees and Magnepans cancel their side waves, so a formula of 0.618 x the ceiling height can be used for determining placement from the rear wall. Most box speakers radiate low frequencies in all directions thus a formula that places the speaker to rear wall distance at 1.618 the side wall distance should be used.
The distance from the center of the woofer face to the side walls is:
Room Width * .276 (RW x .276)
The distance from the center of the woofer face to the wall behind the speaker is:
Room Width * .447 (RW x .447)
This is all you need to know to place speakers in a symmetrical, rectangular room!