At the start of WWII the main RAF Army Cooperation role was originally filled by the Lysander. Army Cooperation = realtime/near realtime reconnaissance & communication with ground units, observation & artillery direction, night-time flare dropping/target marking, etc. As time went by the RAF used faster and more survivable aircraft (at least for the day-time missions), hence the use of the Mustang in the role as one example.
CAS is usually defined as using the weapons on the aircraft for attacking enemy units close to friendly units - ie on the battle line. CAS aircraft in WWII did not (I think) usually act as observers for artillery, but would often work with ground-based FACs of one sort or another. By mid-war the UK (for example) had specialized OP tanks that filled the roll of artillery spotting and FACs, as well as infantry teams for artillery spotting and FAC.
GA is usually more general in nature. Aircraft assigned to attack point targets (ie railroad trains, bridges, airfields, V-1 launch sites, enemy troop concentrations, etc) which may or may not be close to the battle line and not engaged with friendly units, are usually considered GA missions as opposed to CAS or Army Cooperation missions.