Ok I assume the CustomerId (1) is a parameter of your stored procedure, in my script I have declared one to use. I replicated your scenario and this script gets the output that you need.
Declare @CustomerId int = 1;
Select distinct PM.ProdModelId, PM.ProdMasId, PM.ProdModName, @CustomerId as CustomerId ,
Case when SM.SalesId IS NULL THEN 'false' ELSE 'true' END AS Purchased
from ProductModel PM
Left Join SalesDetails SD on PM.ProdModelId = SD.ProdModelId and SD.CustomerID = @CustomerId
Left Join SalesMaster SM on SD.SalesID = SM.SalesID
I hope it helps
Good luck.
Azee...