Le Petit Bateleur wrote:Bring cards to the top as you "loose them into the deck", palm the three cards, allow the spec to shuffle to their hearts content, and put the three cards back on top?
Palming ain't possible for three reasons, though it makes perfectly sense.
1.  

 and most embarrassing, I am not able to palm cards that it looks natural nor can I hold it in a way that it, won't be seen unless I bend it beyond whats good for the cards. I just haven't invested time in this yet. Sounds stupid, but sleights aren't one of my strength's. (Let's say I am clumsy, when it comes to dexterity, while I managed some basic handling of some very rudimentary sleights, I still have huge problems doing an ordinary riffle shuffle and I don't know why.) 
Though the two more interesting objections against palming in this scenario are
2. I don't want to touch the any of the deck after I handed it to the spectator, for this is a prediction effect.
3. One of the three cards is at the moment at least genuinely choosen by the spectator and just acts as a tool, which marks the predicted cards later.
Hope you can see, how cheap the method is I am currently applying. 
