Sediment filter first, then usually the Granulated Carbon Filter, then Carbon Block Filter.
But three stages are a bit useless actually, it's more marketing then anything else (ie 6 stage filtration etc etc)
What you basically only need is the sedimentatry filter, and the Carbon Block filter. That's how it comes on most 4040s. Plus side to that is higher flow rates, which is essential on a 4040. Wouldn't make too much difference on a 300GPD I can imagine.
If you have any questions on water purification and such, the place to ask them is at Gaps Water, her name is June, and is one of the most knowledgable persons out there. Most people, if not all would agree with that. Very friendly lady.
01706 521074
http://www.gapswater.co.uk/