How do I delete a login item on my MacBook Pro?

What do you mean by "Startup Item"?

If you mean Login Item, then see this Apple article:

Remove login items - Apple Support

You need to be more specific. What menu? Remember we cannot see your computer and people can have a lot of unknown custom configurations which can complicate matters.

Besides the Login Items that you can configure yourself, you can also uncheck "Reopen windows when logging in" from the pop-up window that appears when you log out, restart, or shutdown the Mac. See this Apple article:

Prevent apps and windows from reopening on Mac - Apple Support

I have three items in my menu when I open my computer. I want to to remove one of them