I do three sciences, but i'm only in prelim; I definately wont be dropping though. I know two people from my grade other than me who do three sciences AND extension (1) maths, i even know a few kids (from different schools) who do three sciences, extension maths, and extension english! The point is, none of these guys have a problem with doing all three, i personally find it easier than to be doing three completely unrelated subjects.
But, you must at least LIKE science, there is no way you're going to get through it being a person who hates science but does well in it. That's because if you do three sciences, you're obviously going to have a period of science everyday; for me that's okay, 'coz i need my daily science fix (lol).
So, in a nutshell: Doing three science is by no means hard; that being said, it's only an option for those who don't get sick of it easily, it is not enough just to be getting 'good marks'.