The answer isn't a simple one
since it depends on the configuration of the antennas
on both ends, and what your objective is. Theoretically,
a system like 802.11n that uses three antennas,
depends on orthogonality to realize maximum efficiency.
In an ideal environment (no reflections, equal path
lengths, equal antenna performance, equal TX/RX
circuitry) the three orthogonal antennas independently
transmit/receive energy from the same signal in
all three planes, without any interaction. The receive
system would add the signals.
In the real
world you know that the antenna configurations have
to be adjusted for best average performance in the
operational environment. Same goes for two antennas
or even one antenna.
For any system of antennas,
the net performance depends on spacing between antennas
and how the receiver handles the signals. Again,
for and ideal scenario where all antennas are far
apart enough to be considered independent and all
signals arrive at the same phase (or can be adjusted
for addition), the total received power is the sum,
in watts (not dB), of the individual signals.
To get a meaningful feel for how your intended
configuration will perform, a computer simulation
will be needed.