Views Load More: Showing X number of item initially
Well you could use this workaround. 

Have a view with the initial 3 (#1-3). Then add another view below it with an offset of 3, so it shows #4-6. Thus, it will load 3 rows at a time. 

You could just show the views back to back on the page or you could use the Insert View module, and have it load the #1-3 view at the header of the 2nd view.