Complex sentence: In the Far East the M3's main role was primarily of infantry support, and they played a pivotal role during the Battle of Imphal, despite their lower-than-average off-road performance they served well on the steep hillsides around Imphal.
Split into simpler sentences: In the Far East the M3's main role was one of infantry support. They played a pivotal role during the Battle of Imphal, and despite their lower-than-average off-road performance they served well on the steep hillsides around Imphal.