In the Elo model, C would be over one class interval (+240) stronger. B the average player (=0) and A over one class interval weaker -(240). The Elo model predicts a win for C against A with 95% probability (rating diff = 480).
See here for the rating probabilities: https://www.fide.com/docs/regulations/FIDE%20Rating%20Regulations%202022.pdf.