Connecting my devices with Mac as the "server" I have three devices : A MacBook Pro (i7, Retina, 2014 one), Xiomi mi4i, and Apple iPad (4th generation).

So my issue is every time I have to transfer and access files, owing to fact that only two are of the same ecosystem, it is quite a problem.

I recently tried using BitTorrent Sync (for about two weeks now). The problem is that it works only 30% of the time , and is other wise very laggy. In addition to this I have tried Share IT, Pushbullet , Instashare. 

USB is an option, but my requirement is to use my Mac as a repository for the files, and access as I need. Is there a way I could set up a network between my three devices (by say smb or ftp) and then access files (like the BitTorrent way) ? I did try the share folders option, but I didn't quite understand it correctly.
If you share files from your Mac by checking the box next to 'File Sharing' in the 'Sharing' pane of 'System Preferences' then you need to connect to that share using a client app on your device (like the 'File Connect' app for iOS.) Apple File Protocol (AFP) and Microsoft compatible SMB are the two choices for sharing.

Bittorent is a peer-to-peer method of sharing files. AFP, SMB, and FTP are client/server methods of sharing files and work completely different from peer-to-peer. With a client/server method, you would access your files directly from your Mac.

For what it’s worth, if DropBox is not working out for you, the problem is likely on your end.