It was a four carriage train. I think they are using plat 3 for the shuttle trains stopping Brighton/Falmer/Lewes/Falmer/Brighton. I will check tonight if they are continuing this service.
They will probably use Platform 3 in the evening games - to be fair it can take longer trains eastwards I think, just not westwards - because there aren't so many evening services to/from London to hold up as there are on a Saturday afternoon. Once Brighton itself loses all the Summer holiday visitors as well that might make things a bit easier to cope with - but for 22,500 max...