1) it's becoming more popular every day.
2) it's becoming more popular each day.
3) it's becoming popular day by day. this one is ok. It means it's gradually becoming popular. But "more" would be better here too.
1. And 2. In the original suggest that it becomes popular one day, reverts to being unpopular at night, and becomes popular again the next day, and so on.
can someone please tell me if the above sentences are natural? If yes, what's the difference between them?