Is it ok to write back when no response has been received from a postdoc application? This may be a very general and subjective question, I know. But, I have applied to some postdoc positions, and after 3 weeks I haven't received any response yet. I suppose this is normal, because many people should have applied to those positions, too. However, I don't know if it would be ok to write back to the professors or PI to know if they have already a response for my dossier. 

Does anyone could tell me if it is fine to do so? should I just be more patient and wait more time? after how much time should I consider my application as rejected?. 

I would be very glad to read your suggestions, and experiences. 

Update: In my experience I think that writing back is fine (there is nothing wrong about it), but it is likely no response will be received anyways. In general, they reply in two situations: 1) they are interested in your profile, 2) they have funding.
During academic job applications, I did not hear back at all from at least two places (in the UK), so I would not assume you will hear back. On the other hand, there was also the situation where I had not heard back from one of my top choices so I contacted them and they told me that someone was supposed to have contacted me two weeks earlier but somehow didn't. It wasn't good news, but I had been stalling on agreeing to another position so ultimately it was helpful to just ask, politely obviously. I really don't think it would hurt your situation, and as others have mentioned, may even help it.