Depends. A3s were built off a mk4 platform up to a certain age (think 03) then after were built on the mk5 platform.
i belive (stands to be corrected) that the mk4 dash fits in the a3 built off the mk4 platform but not mk5.
as for the leon your guess is as good as mine

however i agree. have mates with leons and a3s and everyone prefers my dash.
my mate even has a leon k1 which i though would have a brilliant interior. how dissapointed i was when i got in it haa