Views - sticky items per page paginated using several displays (Hooks or UI way)
First off, this can all be done in views, no custom code required (yay!). This is accomplished by creating an attachment view which will inherit the pager settings.

First you create a view with a page display that shows 7 Y nodes only.

Second, on the same view, add an attachment display. For the attachment, set it to show 3 X nodes only. In the attachment settings, make sure that Inherit arguments, Inherit exposed filters, and Inherit pager are set to yes. Set Render pager to no. Set Position to before and set Attach To to your page display.