Hi Jean,

Attached is the sample file. 

There are a few ways to do this. If you look at the component APIs help 
<https://support.google.com/webdesigner/answer/6279926?hl=en> page, it 
shows that the pageDeck has a method goToNextPage. You can set the custom 
action in the first page to go to the next page as followed:
Event: page > ready to present the page
Custom action:
setTimeout(nextPage, 1000);
function nextPage() {
  document.getElementById('pagedeck').goToNextPage();
}

Then in page 2 and 3, you can use the same custom action.

In page 3, you will need to add a new custom action to go back to page 1 by 
using the following:
setTimeout(function() {
  gwd.actions.gwdPagedeck.goToPage('pagedeck', 'page1', 'none', 1000, 
'linear', 'top');
}, 1000);

If you don't use gotoNextPage method, you can add separate custom action 
for each page to use goToPage.

Hope it helps.
San
Google Web Designer team