1/3 cut means that there are three possible tab positions, each being 1/3 of the width.
1/5 cut means there are five possible tab positions, each being 1/5 of the width.
I guess what you choose really depends how much writing you need to fit on the tab.
I can't find 1/3 cut manila folders in A4 size in Australia. Seems that 1/5 cut are all you can get. However you can purchase 1/3 cut on eBay from the USA at quite a reasonable price. They are however "Letter" size rather than A4 size.