Almost all ASP.NET web forms return to the same page. If you are not working on some weird outlier (which your question does not indicate), you simply use one form tag around the entire document and use event bindings to your controls to handle various instances.
For example, you may have three buttons that act as submit buttons, one for each "form", but everything inside this tag form, and each of them triggers a corresponding event. This behavior is standard and is handled by ASP.NET for you; all you have to do is write event handlers and bind them.