I don't know why you would want to run different sized tires on an MP3 ... You forgot to post the all-important offset, but I think 205/45/18 is too big no matter what.
for tire size....you will need to run a 215/35/18. other wise its going to be not only too tall...its going to rub.
and like mentioned.....the most important piece of information is missing...what is the offset of the wheels you are looking at...
for the MP3....if that wheel is 7 inches wide you will need a +43 mm offset....if its a 7 1/2 inch wide wheel...you will need a +45 mm offset....otherwise you will rub.