To my knowledge (which is pretty good with Southern operations), the only three way splitters were Victoria to Redhill, which then split into Reigate/Tonbridge/Horsham. The only other one that is sort of a three way split, was the Victoria to Littlehampton and Ore, which split at Haywards Heath, but then during the evening peak (and possibly just late at night) dropped off four coaches at Eastbourne. So if you were going to Hove, you may have to have been in the front or rear four. Bit more tricky when you wanted Ore and you had to be in 5,6,7 & 8 as 1-4 were dropped at Eastbourne and 9-12 carried on to Littlehampton.
I wouldn’t be surprised to know something similar existed where 12 coaches went to Haywards Heath, four came off for a run to the East Coastway, four went fast to Brighton and four carried on via Hove. An example, but something like that could have easily existed near the end of service only a Friday or something.