Three!? Seriously!
I could never narrow it down to just three, even 10 wouldn't be able to cover all the classical music I rate equally highly. So, I will have to cheat. 😉
I am only into: 20th century, atonal, serial, avant-garde classical music. Mostly from post 1950 to the present.
Alban Berg - Violin Concerto
Elliott Carter - Concerto for Orchestra
Charles Wuorinen - 4th piano Concerto
Joan Tower - Concerto for Orchestra
Ernst Krenek - Static and Ecstatic
Magnus Lindberg - Sculpture
And a few, less 'thorny' sounding pieces:
Bartok - Music for strings, percussion, celesta
Samuel Barber - Piano Concerto
Stravinsky - The Rite of Spring
Again, there are many other pieces that I rate as highly as those I listed. If I made this list tomorrow, it could be almost completely different.