You can combine standard forms, one of each type, on one web page:
Email sign-up form
Interaction form (volunteer activity or constituent information)
Transaction form (online giving or event registration)
For example, your web page can have a email sign-up form and an online giving form on the sidebar. It can also have a volunteer activity form on the main section of the page. The page has one of each form type.
However, your web page cannot have an online giving form on the sidebar and an event registration form on the main section of the page. The page cannot have two transaction forms.
Also, custom or self-hosted forms may not work when combined with other form types.