Hi Rladeira, welcome in the fascinating world of PSoCs!
PSoC3, or 5 contain only digital filters and no instrumental amplifier at first sight. Depending on your requirements (Voltages, frequency, noise) you may use an external amplifier or you build one with the opamps contained in the PSoC.
Nevertheless there are LPFs but their use is a bit limited.
Another choice would be to go with a PSoC1 which has got internal analog filters, amplifiers which could be used as instrumentation amps and ADC. The downgrade is speed and debug capabilities which are inherent for PSoC3,4,5 evaluation Kits but for PSoC1 only with an In-Circuit-Emulator (ICE-Box).
The importance of the IA in a design is crucial. One can "easily" delude oneself
into thinking I have a 20 bit A/D, all is good, when in fact the IA is the front end
and source of most of the errors in high resolution/precision/accuracy designs.
Read the thread carefully PSoC73 mentioned, as it shows a lot of NON WORKABLE
Your choice to get a "reasonable" IA out of PSOC is to use -
1) A/D in diff mode, but it is limited as to its input buffer G choices.
2) Use OpAmps in PSOC + precision external R network. This is "OK" at DC, but
if you need AC performance is compromised by phase delay matching, and layout.
3) Use external IA. from the likes of NSC, TI, Analog Devices.
This might help, well worth the read -