Can I use Mac mini as a fileserver to share files between iMac and MacBook Pro?

Yes. In the Mac Mini, enable File Sharing. Go to System Preferences (aka Settings) > Sharing. Enable File Sharing. Also there, under Options, enable "Share files and folders using SMB." Disable "Share files and folders using AFP." For good measure, restart your Mini after doing this.

The Mini should then show up in the Finder on your other Macs. You can then log into the Mini from the other Macs with whatever Mac user ID(s) & password(s) are on the Mini. This also enables sharing of any external drive(s) that are connected to the Mini.

I have an old 2010 MacMini that I use for exactly that purpose. I not only use it for a fileserver but also as a place for additional backups of data and photos and as a media server using Apple Home Sharing. Although being a 2010 with only USB 2, moving large amounts of data is a bit slow but it still serves its purpose.

You should be able to.