Q: Create a public folder

Try screen sharing. Log in as the admin account on the other computer.

Screen Sharing – Share the Screen of another Computer

Your Macs are already on the same local network and therefore you have a couple of options.  I am going to assume that: 1) you are the only user that you are concerned with sharing, and 2) your username and password are exactly the same on all 3 Macs.

Option 1:

1. Find the "name" of each Mac at: Apple Menu > System Preferences > Sharing > "Computer Name"
2. On each Mac, click Apple Menu > System Preferences > Sharing > enable "File Sharing"
3. In the Finder sidebar of any other Mac, look under "Shared", click the name of a shared Mac, and then "connect" in Finder.
4. Choose to connect to your username and then you will have full access to your entire user account and all of it's files.

Option 2:

1. Create an account and install Dropbox (dropbox.com) on all three Macs.  This will create a "Dropbox" on each Mac that will immediately "sync" the same documents as changes occur.  You can learn to use the "selective sync" feature if you have problems with small SSD hard drives on laptops.

Actually the Mac OS already has your scenario covered.  Each user account has a "Public" and "Public/Drop Box" folder pre-established for these types of file exchanges.  You can place files that should be available to your wife in your "Public" folder, and she can drop files to you by placing them into your "Public/Drop Box" folder.  Note that the "Public/Drop Box" folder has no relation to the DropBox service so don't let that confuse you.  Here is an FAQ on how it works once you have enabled File sharing on each Mac:

OS X Yosemite: Access another Mac user’s Public folder and Drop Box

You can also use the DropBox cloud service in a similar way.  Login to the DropBox web interface.  Then create a folder for your Wife and use the sharing tools to share it to her DropBox user.  Then when you place files in that folder she will have access to them from her DropBox account.

You are looking for a folder that already exists at "/Users/<your account>/Public".

Open Finder, then click the "Finder" menu > "Preferences" > "Sidebar".

Make sure that under "Favorites" you enable the box next to your account name.  Then, in the finder sidebar, click your account name, then look for "Public".  When you see it, drag "Public" to the Finder sidebar for easy access.  Within "Public" you will see the "Drop Box" folder where you Wife would be able to drop things to you from her account (locally or from a remote machine via mounted share).

If it's asking for password then you likely don't have a matching account for you Wife on the iMac.

Make sure that you have created a matching user account for your Wife on the iMac as well.  To connect from one Mac to the file shares on another, both Macs need to have the same user account so they can authenticate that the person connecting should have access to the share.