Hi Alex and welcome.
I'm also in the UK and have a similar coop for my bantam pekins.
It is definitely going to be on the tight side for your girls particularly roost space wise.... Light sussex are big broad birds and come winter when it's freezing you want them to be up on the roost not standing on a cold metal tray where they may get frost bite on their toes. The current roost space is definitely not sufficient for all 3. That said, light sussex are reasonably mild mannered birds, so if you can increase the roosting space and increase the run area with an add on section, you might get away with it, but it definitely needs some modification to even consider it longer term, for your 3 girls. It also needs to be located in a sheltered spot as they don't like wind blowing through the run.
I fitted two half round fence posts to the bottom of the long sides on mine to act as skids, so that I can drag it along the ground and give them fresh grass each day although at the moment they free range the lawn. The fence posts also provide more rigidity to the construction.
Good luck whatever you decide.
Regards
Barbara
I'm also in the UK and have a similar coop for my bantam pekins.
It is definitely going to be on the tight side for your girls particularly roost space wise.... Light sussex are big broad birds and come winter when it's freezing you want them to be up on the roost not standing on a cold metal tray where they may get frost bite on their toes. The current roost space is definitely not sufficient for all 3. That said, light sussex are reasonably mild mannered birds, so if you can increase the roosting space and increase the run area with an add on section, you might get away with it, but it definitely needs some modification to even consider it longer term, for your 3 girls. It also needs to be located in a sheltered spot as they don't like wind blowing through the run.
I fitted two half round fence posts to the bottom of the long sides on mine to act as skids, so that I can drag it along the ground and give them fresh grass each day although at the moment they free range the lawn. The fence posts also provide more rigidity to the construction.
Good luck whatever you decide.
Regards
Barbara