It depends if the Guitar Hero 3 songs were released as DLC on GH: World Tour. There is no way to directly import songs from Guitar Hero 3 onto World Tour.
I don't think you can.
Only in Multiplayer.
NO
no
NO
26 songs.
Guitar Hero World tour "Beat It"
You need to import them
Yes, there are downloadable songs for guitar hero world tour.
You beat all of the songs in that venue (getting 3 stars at minimum...) Once you beat all of the venues and career modes,(lead guitar, bass guitar, guitar duel) you get the last 3 bonus songs.
no.. not at this time