Complex sentence: Each company was equipped with just one light mortar, rather than the usual three, and their supporting arms, such as engineer and signals elements, were only of company size, rather than the battalion-strength elements included in front line formations.
Split into simpler sentences: Each company was equipped with just one light mortar, rather than the usual three. The supporting arms of these divisions, such as engineer and signals elements, were only of company size, rather than the battalion-strength elements included in front line formations.