Sorry Raves, but I disagree. Here is why.
Pok, the fact that you are working on 3 handed casting is very likely the reason your dog "Popped" that is what your dogs action is called. It can become a very big problem. Unfortunately you did the worst thing you could have done in that situation!!!!!
If a dog "Pops" , the last thing you want to do is "handle" the dog to the "MARK",(you just re-enforced the dependence on you) rather, when a dog Pops you should just stand there and basically ignore the dog, don't speak, don't wave your arms, just stand there. Most dogs will learn that if you don't give direction then they must find it on their own and will re-engage in the "hunt". NEVER handle a dog on a Mark! If he refuses to re-engage after a period of time call him to you (or if he simply returns to you on his own) Speak to him in a rough angry voice letting him know you are displeased, make him SIT and then YOU go pick up the Mark yourself! Do NOT allow him to tag along, make him wait at the Sit position.
When you return, (disapline is over) re-position him and throw another Mark to a new fall area, make it an easier Mark to gain his confidence. Continue with Marking drills, increasing difficulty slowly over the next few training sessions.
Keep your 3 handed casting to separate training session and in a different area if possible, until he is good and solid with that.
Much later in your training program you will incorporate Marks and Blind Retrieves together during the same session, but not yet! Good Luck I hope this helps.