>>> do we need to have three external antennas connected to radio 1 to benefit from this setup?
=> No; if you search for antanna-options for this MSM, you will find the two-antanna-option(s).
>>> what if we just use 2 antennas on for radio 1 on ports B & C, will we have lesser transmit power
=> no Rf power is the same,
multiple ntanna's are used for "diversity" technique.
>>> and receive sensitivity for radio 1?
=> Yes, This diversity technique is a way to
make the antenna's "directional sensitive" (per RF-client), so the three-antenna setup is (supposed/designed to be) more sensitive.
But then again external antenna's are more flexible to setup. So this setup can be more sensitive then using built-in antenna's.
It will be hard to be predicted the result in advance.
look at doc's in this link
http://h10144.www1.hp.com/customercare/support/manuals/antennas.htm