in a nut shell......
WPI- is absorbed really quickly by the body... being partly broken down it loses soem of its micronutriiotnal value as apposed to WPC. probably like processed by the liver within 30-45 mins - this is what most serious people will use post workout to get po\rotein in quick....
WPC- Good all round protein.... not quite as quickly absorbed as WPI , contains more carbs, is creamier.. its cheaper and the best opition for 90% of people unless they are anal about carbs
Casein- slow release protein up to 6 hours for full release. most take this at night before bed so they dont go into a catabolic state whilst asleer.
there the new hydrolyzed WPI aswell which is even quicker absored then WPI.
mixing all 3 doesnt really matter.. but theres no benefit post workout of having casein over WPI/WPC... you are better off having WPI after a workout then eating a meal 30 mins later..