Here is who I can see singing what Britney songs I imagine. (S3 Major Characters Only)

Lucky by Britney Spears. Sung by Rachel Berry (Sung About Finn)

Circus by Britney Spears. Sung by New Directions (The Always Needed ND Number)

Hold It Against Me by Britney Spears. Sung by Tina Cohen-Chang and Brittany Pierce (Girls Duet)

I Wanna Go by Britney Spears. Sung by Blaine Anderson and Artie Abrams (Boys Duet)

Oops I Did It Again by Britney Spears. Sung by Quinn Fabray (backup by The Unholy Trinity) (Sung to Puck)

My Prerogative by Britney Spears. (Originally by Bobby Brown) Sung by Finn Hudson (Finn always gets his number)

Womanizer by Britney Spears. Sung by New Direction Girls

Sometimes by Britney Spears. Sung by New Direction Boys