First measure the full size tire, then measure the A3 spare tire well, compare these two measurements closely. If for some reason you can subtract one from the other and get a positive or negative number, then there is a possibility that they do not match, ie it won't work.
However, if you cannot subtract one from the other and get more or less than Zero, then take the full size spare and place it in the spare tire well.
When it fits in nicely, post the results here. BTW, didn't your A3 come with a full sized spare to begin with?
My NB and Jetta both have FULL sized spares, another reason why I purchased a VW instead of an oriental import or American domestic, they almost all have these pathetically useless skelton spares in the trunk. Those skelton spares sure work good, until you need them, then they suck. The full sized VW spares work great as long as they have air in them.