I think it's only one an hour though, leaving on the hour from Gatwick and getting into Brighton on the half hour. Worth double checking closer to the day though :)
None between 12:20am and 5:32pm on the 26th from Three Bridges to Brighton as far as I can see. The trains from Victoria aren't stopping at Three Bridges it seems.
Edit: Victoria to Brighton trains are stopping at Gatwick though, but I don't know how easy it is to get to Gatwick if public...