That is crazy mate....giving you 2 days to prepare is pretty terrible (i hope you've did a bit of prep before hand to make it not so difficult for you). I really do think though that a big part of the interview is on your motivation for ATC...and I think 'I only got 2 days notice so didn't have enough time' would sound pretty bad in an interview - even though it's a completely valid response. Is there no way at all you can arrange a visit for the day before your interview? If not...do you know any Air Traffic Controllers to phone and ask/interview ? If it were me...I'd e-mail them and ask when a different stage 3 date will become available - if you've made the journey THIS far...there is no point in throwing it away on the final hurdle. So you have to wait a little longer...it gives you so much more time to prepare, and that is only a positive thing!
All the best mate!