I'm thinking about using class (singleton) with collections implemented using ConcurrentDictionary. This class will be used as a cache implementation (asp.net / wcf).
What do you think about exposing these collections explicitely from such class vs exposing just e.g. 3 methods (get,add,clear) for each of them (using safe methods from CD) ?
As you're implementing a cache then I'd suggest only exposing those methods that you need to the outside world, to prevent any unexpected side-effects that result if a.n.other user fiddles with the dictionary.
A cache without an expiration policy is a memory leak. Whatever policy you come up with is going to affect the Add() method. You have to wrap that method to initialize stuff that lets your expiration logic work. You can't just let the client code manipulate the collection directly.
So, yes, create a class wrapper.