Why use a three opamp instrumentation amplifier?
The two op-amp design has in principle some drawbacks in comparison with the three op-amp design: the common mode input range is lower and the matching of the resistors is more critical if a high CMRR is to be achieved (Graeme, 1973). However, these drawbacks become less important at high values of overall gain


http://www.biosemi.com/publications/artikel7.htm

On the other hand, this claims the CMR is better for the two-op-amp version:


  The two-op-amp configuration can provide higher CMR, especially in low-voltage, single-supply applications.


I'm not sure which is correct. Also:


  The V1 signal must propagate through two op amps, but the V2 signal propagates through one op amp. When input signals contain frequencies greater than the flat portion of the op-amp gain curve (Reference 2), the V1 signal attenuates more than the V2 signal. The unequal attenuation causes the signal to unbalance, and CMR reduces at high frequencies.


http://www.edn.com/article/492092-Don_t_fall_in_love_with_one_type_of_instrumentation_amp.php#ref