The Preview and Save & Email buttons should also work automatically after clicking the Generate Document button on the quote object. How can i customize those three standard buttons as one custom button in CPQ Quote object.
Unfortunately there is no way to customize it as Generate Document page is managed component part of CPQ package. There is another questions on similar lines: CPQ How to Hide Preview Button
You can't modify the buttons directly on the generate document screen. What you could do is create a custom LWC, flow, or button on the quote object that uses the CPQ API to generate the document.
Note that there are some limitations with generating through the API.
- Attachments marked Required are ignored when you’re using the API. In contrast, when you’re using the Preview and Generate document buttons, the attachments are automatically generated as part of the document.
- An attachment in the Template section is always included regardless of whether it’s marked Required. This behavior is also observed when you’re using Preview and Generate document buttons.
- Guest users can’t create quote documents in the Documents folder. See Salesforce Help and SOAP API Developer Guide for details.