Gather all the keys to the car and remove any loops or rings from the keys.
Sit in the drivers seat with all the keys and all the doors shut, radio off, no lights, everything OFF
Insert a key into the ignition
Turn ON the ignition
Press and hold the LOCK button on the key for TEN seconds
Turn off the ignition and remove the key
Insert the next key and repeat the procedure
WAIT 1 minute, do NOT switch anything on.
When the 1 minute is up, open the drivers door, exit the vehicle, close the door
Test the keys each lock and unlock the doors.
If the keyfob battery is flat you need this CR2016 battery (click here) they last for years on the shelf (or in the drawer).
If you have the facelift or new shape C3 (2006 and later), you will need a CR1620. You will probably get a message 'SERVICE' or 'Plip battery exhausted' message which will need to be cleared. See the instructions to clear the Plip battery exhausted message (here)
Get the battery here to fix your 2206 an on Citroen C3 fob
Pop open the key fob with a coin to show the battery and check the number to ensure the correct replacement.
There is a topic for repairing the fob (click here), useful if you have a button or two missing or even a broken switch. Follow the link above to make a repair and save having to buy a new key fob.