Look, if you think Royal Holloway is going to be better, firm it. The reason UCAS is set up the way it is is so that it allows you to have a fall back. If you are confident of getting ABB and think AAA is possible with hard work, you are better off firming RH, working hard and then just seeing. If you end up with AAB or ABB and don't make it, at least you tried and still ended up at a decent university. What would be worse is if you firmed sussex, got AAA and ended up regretting not firming RH.