as far as i know the bolt pattern for w203 c320 is 5 x 112 right ?
so can any body tell me what are the difference between that and 5x114.3. can 5x114.3 fit into my car ?

 

5x114.3 bolt pattern will not fit on a c-class...
if you were to draw a circle connecting all the bolt holes, that will give you the diameter in mm... so a 5x112 will have a diameter of 112mm while 5x114.3 will have a diameter of 114.3mm so it will be off...

check this link off the tirerack website which explains this pretty well... http://www.tirerack.com/wheels/tech/boltpatt.htm