Customizing Slicer 3d toolbar

How to customize slicer in-built toolbar?
I want to hide some in=built buttons in toolbar and add custom buttons into it?

In general, you would hide all toolbars except your custom toolbar, using slicer.util.setToolbarsVisible(False, [myToolbar1, myToolbar2]). You can add your custom toolbar and actions to it using standard Qt calls.

Can I hide certain widgets like(marksups) in the built-in toolbar,without hiding the whole toolbar I just want to hide some of them?

This is all just standard Qt API, so you should find answers by googling qt hide toolbar button. If you have any specific issue then let us know.

How can I get the toolbar instances?

This would also be part of standard Qt API. Widgets have children and parents. So you will need to familiarize yourself with and type calls for finding various Qt objects through the application.