Our household has 2 iphones, a new iMac and a laptop. The 2 phones have their own apple id. Can the computers share one of these?

If I am understanding your question, you can use the same Apple IDs on the computers as you use on the phones. There isn't a need to create a separate Apple ˆD.

The organizer is the primary in charge of Family Sharing.

You are welcome. I suggest that instead of changing the Apple ID used on the iMac, you create a new user account so each person has their own account on the computer. That way both of you can use the computer, sign into iCloud, and other areas, but keep your data separate. Like having the two phones on the computer and still separate. You might want to do the same thing on the laptop. Having two admin accounts is good if troubleshooting becomes necessary.

https://www.lifewire.com/add-administrator-accounts-to-mac-os-x-2260717

https://www.lifewire.com/create-new-user-account-mac-2260738

Family Sharing.         

Family Sharing - Set up.       

Family Sharing - parent or guardian designate.     

Family Sharing - Use your own Apple ID for Family Sharing

Family Sharing -What types of content can I share?     

Share App Store, iTunes Store, and Apple Books purchases with Family Sharing