I would really miss having:

Tegan and Sara - The Con
Ida Maria - Oh My God
Rammstein - Du Hast
Paramore - Misery Business

If I had to choose 3 they would be:
Plush
No One Knows
The Look

With "Just Like Heaven" and "Here I Go Again" being a close 4th and 5th place.

Just Like Heaven (My most played by far), Bohemian, and Roundabout or Sister Christian for me.. prob my most played..the others do nothing for me and never get played.

Too many to mention. Off the top of my head:

Saturday Night's Alright For Fighting
Lasso
Stop Me If You Think You've Heard This One Before
Just Like Heaven
Bohemian Rhapsody
China Grove
Yoshimi Battles The Pink Robots, Pt. 1
Sister Christian
Imagine
Low Rider
King George
I Can See For Miles
Break On Through (To The Other Side)
Crosstown Traffic
25 Or 6 To 4
Misery Business
Centerfold
20th Century Boy
Rock Lobster
Me Enamora
Combat Baby
Everybody Wants To Rule The World
Whip It
Werewolves of London
The Con
In A Big Country
Don't Stand So Coose To Me
The Look
The Killing Moon
I Wanna Be Sedated
Smoke On The Water
Here I Go Again
Walk of Life
Space Oddity
Walking On The Sun
The Power of Love
No One Knows
Get Up, Stand Up
Good Vibrations
Heart of Glass
Plush
Get Free
The Hardest Button To Button
Free Bird
Foolin'

In no particular order. Heck, I'll even miss "Fly Like An Eagle" a little bit.

That's just off the top of my head. There may be more. And if you tell me to cut that down to three.. heh, yeah right.

I would miss these songs something fierce:

The Cure - "Just Like Heaven"
Blondie - "Heart of Glass"
Tears for Fears - "Everybody Wants To Rule The World"
Metric - "Combat Baby"
Paramore - "Misery Business"
Echo & The Bunnymen - "The Killing Moon"
The Smiths - "Stop Me If You Think You've Heard This One Before"

I'd miss "I Can See For Miles" the most.

25 or 6 to 4

Holy crap would that be sad. And the fact that we haven't gotten any more Chicago since RB3 was released makes me a bit worried...