for 2 years now ive had RO 40 40 with 2 pre filters, i normaly just get the 2 pre filters from veritech.
but ive found some on ebay.. 1x 10" 5 Micron Sediment Filter
&
1x 10" 5 Micron Carbon Block Filter
&
1x 10" Carbon GAC Filter
can you please tell me if i need all three ? will this bring the tds down even more before RO 
I have never used a GAC filter in the years I have had my 4040.
wintecs used to be the cheapest for 10inch filters, the ones your talking about are fine.
Changing pre filters every two years is way to long a time period your water must be trickling out.
it will not likely to take tds down but will take out the things needed to protect your membrane (which is the expensive bit inside)
you need the first 2 for sure.