PBKDF2, scrypt and bcrypt are all configurable; they can be made as slow as you want. The limiting point is not the computer, but the user's patience.
For example, suppose that the user will go irate if the password processing (e.g. to unlock an archive file) takes more than 6 seconds. If you use bcrypt only, then you can tune it up so that it takes 6 seconds on your machine. But if you use PBKDF2 and scrypt and bcrypt in succession, then they will have to fit together in the 6-second budget, meaning that, in practice, you'll configure them with lower iteration counts, e.g. 2 seconds each. From that point of view, using three algorithms in succession is no better than one; what matter is the total time.
The point of this configurable slowness is that it makes things harder for the attacker; you want to choose an algorithm that makes each password guess as expensive as possible for the attacker. The attacker does not have necessarily the same hardware as you (e.g. he will likely try to use a GPU), so each algorithm may be more or less "good". The attacker can always buy a PC such as yours, and thus do in 6 seconds what you do in 6 seconds; however, the attacker may sometimes buy a more specialized hardware which will be better than yours for that specific task. This is especially the case with PBKDF2 and GPU. As a defender, you want to avoid such a situation. Thus, some algorithms are better than others, and, in the PBKDF2 / bcrypt / scrypt triad, one may assume that one algorithm is "stronger" in the sense explained above.
Therefore, using just that algorithm, alone and with its configuration adjusted so that it takes as much time as you can tolerate, will be optimal. By definition, using all three algorithms in succession cannot be as strong as that unique algorithm alone. In practice, this means that you should use bcrypt: the budget you spend on PBKDF2 is squandered security, since, for the same CPU usage on your machine, bcrypt would have provided more resistance (see this for some details about bcrypt vs scrypt, and that for even more details on password hashing in general).