Hi Vivo,
Have you got the wheel? and have you got the original gk76A discs? because if so you will find the discs dont fit as the hole centres are different. Google 'metal gear discs' and if you go onto there web site and serch under manufacturer for Suzi 400's the list will allow you to see the disc sizes/measurements and the gk76a are not the same as the 73/B4 that have the 3" wheel. If you are not happy i would inform the seller that you are putting the matter in the hands of ebay. make sure you keep all the emails as ebay will ask for them.
Chris.

yeah send it back mate, you dont want a 3 inch wheel on there, shame ive just sold a 3.5

and there was me thinking the only difference was the width.

Thanks for giving me some more info. Should have enough to convince ebay the seller has got it wrong.

Its a pleasure, ive had Suzis fo rover 10 years and Know them like the back of my hand!