There's engineering work between Three Bridges and Brighton all that week. If you're travelling from London, you'll need to change onto a replacement bus from Three Bridges to Brighton or take the one direct train an hour which runs from Victoria via Horsham and Littlehampton to Brighton and takes 2 hours 12 minutes. If you're thinking of travelling via Lewes, you'll still need to take a replacement bus from Three Bridges.
Trains between Lewes and Falmer and Brighton and Falmer are running.
The Amex is a difficult ground to get to at the best of times, and even more difficult to get away from, and this only adds to the problems.