Chelsea will take on either SC Braga of Portugal or Servette FC of Switzerland in the Conference League play-off round.
The first-leg is due to take place on August 22 with the return fixture on August 29.
Kilmarnock will take on Copenhagen or FC Baník Ostrava (CZE) in the Conference League play-off round, should they beat Tromso of Norway.
Derek McInnes’ side dropped into the competition after losing 2-1 to Cercle Brugge in their Europa League qualifier.
St Mirren, meanwhile, will play Romania’s CS Corvinul 1921 Hunedoara or FC Astana of Kazakhstan in their play-off should they progress past Brann. Stephen Robinson’s side had already beat Valur of Iceland 4-1 in the second-round qualifier.
St Mirren and Kilmarnock are both at home this Thursday in their third-round qualifiers before their second-leg away from home next Thursday.
