The ECM reset procedure consists of turning the ignition off and removing the BACK UP (radio) (7.5A) fuse from the under-hood fuse/relay box for ten seconds. (Note: This procedure triggers the anti theft feature in factory radios and wipes out settings in aftermarket units. You will need the code for the factory radio.)
There are a couple of other ways of doing an ECM reset as well.

I noticed you asked how this code can affect your car. Hard starting, low power and engine stalling are some symptoms of a faulty map sensor. If you haven't noticed any symptoms from CEL 3 then try and reset the ECM.