My other car is a MINI!!!!
I think you need to be a little more specific mate.
Which model of leon did you have? I'm guessing a 180bhp AUQ 20vt?
Which A3 have you got? a pre facelift AGU or a facelift AUM?
There's no such thing as 18mm discs for the front..... it's either 288x25mm on the A3, or 312x25mm depending on which leon it is.
Front wheel hub will probably fit, but only if it needed replacing, same for the o2 sensor as well.
Many parts are interchangable, just need to do a bit of research into it, and be more specific when asking questions