Did carol and Lisa from the saddle club die?

No. Lisa and Carole the original actress that played these character got too old to be in the Series 3. All of the characters have been replaced by new younger actors/actress

Jessica Jabobz who played Melanie in Series 2, died 10-May-2008 aged 17, after fallen onto the rail tracks and got hit by train.

Another younger girl played Melanie in series 1