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.