What's the difference between the three wireless antennas in MacBook Pros?
In general, WiFi uses the same antenna for both transmitting and receiving.

In general, multiple WiFi antennas are used to get higher capacity (i.e. higher bandwidth). For example a common setup is 3x3 MIMO (multiple-input and multiple-output) where 3 antennas are used to exploit multipath propagation in order to increase the bandwidth available.

In very rough terms the idea is that there's a limit to the data rate, you can send/receive at a certain frequency with the technology available. Instead of increasing the data rate, MIMO uses multiple antennas to send multiple data streams at the same time on the same frequency, but from different antennas. 

The receiver can then through advanced signal processing seperate the three signals even though they're sent at the same frequency, because they arrive at the receiver in three different directions.

This idea makes it possible to reach higher data speeds over a 3 antenna link than one with just 1 antenna.

Regarding 2.4 Ghz vs. 5 Ghz - it is the case that some MacBook Pro models use one antenna for 2.4 Ghz and the two others for 5 Ghz. Other MacBook Pro models use all antennas for both frequencies.