What do you mean by change diamonds - do you mean change the melee, or change the centre to a diamond? The latter wouldn't be hard, I shouldn't think, if you find a diamond of the right proportions.
resizing - depends on if the diamonds go all the way round the shank or not. Sometimes if they leave a small bar of metal you can size up or down a little, but three sizes may be pushing it... hopefully someone (or better, your jeweller) can tell you more.
Honestly, I think it would not be worth it to buy that setting but have all the melee re done. The cost of the melee plus the labour would probably be more than the setting itself. I would probably start from scratch myself and make sure it was all done right.