Simply no. I would say that Cichlid are far to aggressive and gregarious for Discus. You can though put dwarf cichlids in with them, ive seen some nice Discus tanks with Apistos and Rams etc.
I think 3 Discus in a 120 litre is already a little cramped as Tattoodfishfan said, i wouldn't reccommend adding more fish and certainly not any African cichlids.