Frustrated with Apple File Sharing

Greetings,

#1 is a Mid2007 iMac running El Capitan (10.11.6)

#2 is a Late2012 iMac running Mojave (10.14.6)

#3 is a Mid2014 Macbook Pro running Catalina (10.15.7)

I'm always having trouble with file sharing on my three mac network. I decided to give it one last try to set it up, just spent three hours, up and down the stairs, without success and I'm totally frustrated. I do everything the tutorial websites tell me to do but continue to have connection problems and other glitches.

For instance, the #3 Macbook Pro won't even let me check the file sharing box to turn file sharing ON. (in File Sharing Preferences). Why is that? How do to fix it?

I can connect #2 to #1 and share files but when I try to connect #1 to #2 either as guest, registered user or "using Apple ID" I'm not allowed and told to contact the system administrator. Well, that's me.

I'm still trying to find out what user name and password is used for file sharing. Is it the same as my computer login name and password? If so, why doesn't it work? When I set up file sharing I use the user name that is me. I'm the only one here so I don't want any others to be able to share.

I also can't understand why I can't remove the "EVERYONE" selection in the "Users" column. I have no situations where I would want everyone to share my data. And what are all those other users? Staff?

_fpsd? Can I remove them all except me?

Can anyone help me? I'm willing to read a guide if there is an "Apple File Sharing for Dummies" book out there. I just want to get it set up so I can exchange a few files now and then without having to email myself.

Thanks,

IMacarroni