To load the "menu.qml" file from "main.qml" you have to use 'Loader' element.
This is how you can do...
In this when the timer is triggered after 3 seconds then we are setting the source property of the Loader element with the desired qml file (in this case "menu.qml"). And as soon as the source property is set the qml file will be loaded.
x: 0; y: 0
width: 360; height: 640
qmlLoader.source = "menu.qml"
You can put the above code in the "main.qml" file at the desired place.