The Pelican is the UNSC's primary transport it can transport troops, supplys and vehicles into battle. it isnt a combat craft the only reason it has chain guns and optional rocket pods is for defence when picking up/dropng off its cargo it's weapons can be used for combat however.

As for it in H3 its unlikely because for 1 its too big and would be hard to fly and even if it wasn't it would kindv ruin halo. it just doesnt quite fit.

However use of it in Halo Wars is very likely because of its ability to drop off troops, supplies and vehicles directly into battle. It would be easy to handle in HW's format aswell.