As far back as the 1930’s, Fletcher, Snow, and others showed
that 3 front channels provides a much better depth illusion.
Why? Because at half the angle, the time delay is
smaller (moving the interference up in frequency) and the
higher frequency is more highly attenuated
(reducing the size of the timbre change).
There are other reasons to have at least 3 front channels.
With 3 front channels, and using nearly coincident miking
techniques, it is possible to create a good illusion of the
front soundfield without having depth or elevation problems.
Aside from avoiding the problem of interference,
non-coincident 3-channel miking has other good
effects, specifically a wider listening area,
measured by subjective test to be 6 to 8 times the
listening area available in the 2-channel presentation.