how do i delete email from my macbook pro, ipad, and iphone at once? I use gmail.

If all three devices are logged signed into the same Apple ID, you simply delete the e-mail from any of the three devices. That will delete the mail from all three devices (there is a difference between IMAP and POP mail, but G-mail should work properly.)

Here's how it works: you delete an e-mail from your phone, and the server then removes the same e-mail from your computer and iPad. It may not happen at once - typically, not until the other devices check for mail again. If they check automatically, the mail will disappear automatically. If they wait until you manually check for mail, the e-mail will disappear then.