Greatest milf threesome:
Kim Wilde(ultimate blonde) & Julianne Moore(ultimate redhead)

Greatest twen threesome:
Miranda Cosgrove & Jenette McCurdy (the iCarly girls!)

Greatest "hate" threesome:
Paris Hilton & Kim Kardashian (not much to like with these two but who can forget their "home"videos?

Greatest lovemaking threesome:
Hilary Duff & Miranda Cosgrove (both stupendously cute)
Would be an amazing X-Art video

Greatest Brit posh threesome:
Kate Winslet & Kate Beckinsale

Greatest coloured threesome:
Halle Berry(those boobs) & Jennifer Lopez(THAT ass!)

Love what you've done!
Greatest British posh threesome has gotta be Keira Knightley and Emma Watson!

Using your categories, but for me:

Greatest milf threesome:
Isla Fisher & Sofia Vergara

Greatest tween threesome:
Ariana Grande & Victoria Justice

Greatest "hate" threesome:
Nicki Minaj & Kim Kardashian (SO MUCH ASS THERE!)

Greatest lovemaking threesome:
Ariana Grande & Jennifer Lawrence

Greatest Brit posh threesome:
Pixie Lott & Kate Middleton

Greatest interracial threesome (let's make sure we're politically correct perverts):
Zoe Saldana & Beyonce