We conducted mobility spectrum analysis on a high quality three dimensional topological insulator film of BiSbTeSe2 to extract mobility mu and carrier density n. Top and bottom gates were applied to tune the carrier density on top and bottom surfaces independently. At 1.5K, when the conduction is entirely dominated by the Dirac surface states, we always find two dominant conduction channels (top and bottom surfaces), with mu = 500 - 3000 cm(2)/(Vs) and n on the order of 1 0 12 c m - 2. However, at sufficiently high temperature ( T = 85 K), when the bulk contributes, a third channel with maximum mobility mu similar to 400cm(2)/(Vs) and n on the order of 1 0 11 - 1 0 13 c m - 2 opens. Our data show the feasibility of the method to analyze the different conduction channels in a topological insulator, being also promising for other similar material systems.