I just purchase a Mac Air. It is asking for my Apple ID. I already have an iPad and iPhone. I would like to have the same id but it won't allow me to

Are you looking for your Mac user account ?

Your Mac user account is your login user name to your MacBook Air.

See the following Apple articles:

If you forgot your Mac login password -> https://support.apple.com/en-us/HT202860

Change the login password on Mac -> https://support.apple.com/en-gb/guide/mac-help/mchlp1550/mac

Reset your Mac login password -> https://support.apple.com/en-mn/guide/mac-help/mh35902/mac

If you can't reset your Mac login password -> https://support.apple.com/en-gb/HT212190

BTW:

You are sign in here .  .  .  with your Apple ID & Password.

Assume that the password of iPad & iPhone is the same.

Your Apple ID is the personal account you use with all Apple products and services. It includes the email address and password you use to sign in, as well as the contact, payment, and security details that you use with Apple.

See -> Sign in with your Apple ID -> Sign in with your Apple ID - Apple Support

See also -> Ways to remember passwords on Mac

Ways to remember passwords on Mac – Apple Support (UK)