Take what you like, there's no point in taking a subject just to keep your options open. You need to narrow them eventually. Also, that's a wicked good combination of subjects, no university will look down on those, though obv. you couldn't get into, say, NatSci, I'd guess, since you'd need another science. Definitely good enough for Law at Cambridge depending on the grades you get, and Philosophy is awesome and would go perfectly with those subjects, but unless you're really sure you can do the extra work I wouldn't advise it (I'm assuming you mean taking it as a fifth). That said, taking 5 isn't that bad if you're willing to give up free periods and you enjoy the subjects.
Go for it, they're cool subjects.