  During 2011 February 13 to 15, three Earth-directed CMEs launched in
successively were recorded as limb CMEs by coronagraphs (COR) of STEREO. These
CMEs provided an opportunity to study their geometrical and kinematic evolution
from multiple vantage points. In this paper, we examine the differences in
geometrical evolution of slow and fast speed CMEs during their propagation in
the heliosphere. We also study their interaction and collision using
STEREO/SECCHI COR and Heliospheric Imager (HI) observations. We have found
evidence of interaction and collision between the CMEs of February 15 and 14 in
COR2 and HI1 FOV, respectively, while the CME of February 14 caught the CME of
February 13 in HI2 FOV. By estimating the true mass of these CMEs and using
their pre and post-collision dynamics, the momentum and energy exchange between
them during collision phase are studied. We classify the nature of observed
collision between CME of February 14 and 15 as inelastic, reaching close to
elastic regime. Relating imaging observations with the in situ measurements, we
find that the CMEs move adjacent to each other after their collision in the
heliosphere and are recognized as distinct structures in in situ observations
by WIND spacecraft at L1. Our results highlight the significance of HI
observations in studying CME-CME collision for the purpose of improved space
weather forecasting.
