So basically your reading is 3ppm before and after.
Have you ever calibrated the TDS Meter?
It could be that the meter is not calibrated or it could be that the resin is starting to saturate, you mention that you replaced the filters but did you replace the resin?
Also did you flush the RO unit...
This is basically a Carbon based version of Seachem Purigen
What are you using it for?
If it is for water clarity then there are many options in the line of carbon
If you are using it for Nitrates and Phosphates then there are many better options out there