There is a way to do what I think you want to do.
make three component smartlists one for each genre
for example if you want the final smartlist to contain 50 songs you might do this.
smart list one = 25 songs from your jazz collection
[Media Type]=[Audio] [Genre]=Jazz ~n=25 -[File Type]=[mp3] ~nodup=[Artist],[Name] ~sort=[Random]
Smart list two = 15 songs from classical collection
[Media Type]=[Audio] [Genre]=classical ~n=15 -[File Type]=[mp3] ~nodup=[Artist],[Name] ~sort=[Random]
smart list three = 10 songs from Rock collection
[Media Type]=[Audio] [Genre]=Rock ~n=10 -[File Type]=[mp3] ~nodup=[Artist],[Name] ~sort=[Random]
Final smart list = all three together
playlistid==1068535586,=999028883,=551888403 ~sort=[Random]
you get the final smartlist by selecting add rule and checking off the three component smartlists in the dialogue box that pops up.
Now you have a random list with 50% Jazz 30% Classical and 20% Rock.
This may seem inellegant at first but it is actually much better to do it with components because you can tweak each list untill you get the right combo.
Hope that helps.