Screens usually measure the horizontal height of the content in the landing page. Ie design build in such a way that the content took exactly three of the screen (horizontally), each screen can be a separate unit.
Another example - three different page without scrolling (about us, product, feedback), if you put in one page (landing page), it will be 3 of the screen.
tomas.Remp answered on July 8th 19 at 16:25
The usual orders for the landing pages is writing, referring to the visible area of the screen ... for example ... 1360x768px single slide full screen