How can I delete pictures off my “I” phone 8 without impacting my IPad and my IMAC

As long as you have iCloud Photos turned on in all 3 of those devices, you cannot.

iCloud Photos is a syncing service. Any photo you delete from a device where iCloud Photos is turned on will also be deleted from iCloud and from any other device connected to iCloud Photos. You can potentially reduce the storage space required on a device by turning on Optimize Storage. See: https://support.apple.com/en-us/HT204264

You can’t. iCloud is a syncing service, not an archiving service. If you don’t want to keep them on your phone you need an archiving service, such as Google Photos, Dropbox, box.com, shutterfly, Microsoft OneDrive, Livedrive, etc. All of these have apps for your iPhone that will archive automatically.

For your Mac specifically you can export photos from the Photos app to a folder or folders, but for iOS devices there’s no way to do it.