A higher quality version of the poster:
I just realized that Threepio also has Rey’s staff. How a droid with no arm mobility could use it is beyond me.
So I’m thinking that Threepio isn’t actually using any of the weapons. He’s either carrying or delivering them to Rey and Chewie. Maybe they’re being pinned down by stormtrooper fire and it’s up to poor Threepio to shuffle across the battlefield and give them their weapons so they can fight back. It would be a pretty amusing scene, but I’m not sure why it’s so important as to be on the poster.