We conducted mobility spectrum analysis on a high quality three dimensional topological insulator film of BiSbTeSe₂ to extract mobility μ and carrier density n. Top and bottom gates were applied to tune the carrier density on top and bottom surfaces independently. At 1.5-K, when the conduction is entirely dominated by the Dirac surface states, we always find two dominant conduction channels (top and bottom surfaces), with However, at sufficiently high temperature, when the bulk contributes, a third channel with maximum mobility 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.