I can't deauthorize devices I no longer have. I just want to authorize my present MacBook Pro, iPad and iPhone and get rid of the others that i no longer have.

See Authorization vs. association - Apple Community. You're confused two similar sounding mechanisms. If you use Account > View My Account in iTunes and sign in you should see something like this:

tt2

From Authorize or deauthorize your computer for iTunes Store purchases - Apple Support:

If you run out of authorizations, deauthorize all

If one of your 5 authorizations is on a computer that you don't use anymore, deauthorize it. If you sold or gave away a computer that's used one of your 5 authorizations, you need to choose Deauthorize All on the Account Information page. * Then authorize each computer that you still use.

* I.e. Use the menu option Account > View My Account > {Enter password} > tap Deauthorize All button.

tt2

You're welcome. 🙂

tt2