Fault Code Help

[MDP91]

Right, so my Clio is decatted, runs fine, smells quite strong of petrol but I assume is down to the decat and I'm getting 32mpg mixed driving although mainly round town.
My emissions light has been on, again down to decat I assume. Got my mate to plug it into his handheld code reader and I got the following:

P0420
P0303

Any ideas??

 

[1812lsd]

P0420 = Catalyst Low Efficiency
P0303 = Misfire on cylinder 3

 

[MDP91]

Right ok, what can I do about that lol! cars running fine though?