I suggest making sure you prep the surface of the old cement before pouring the new material to fill in the deep end. Sand blasting and in some applications, chipping the surface, allows a better bond and removes all loost plaster/cement. Also, a bonding layer, such as Multicoat scratchkote, will help adhere the new surface to the old.

Will this new 3' end still be the deep end? If not, there should be some sort of drain at the deepest part of the pool for debris to settle and be sucked out by the pump. If not, I'd recommend that you go a little deeper than 3' so that is it the deep end, THEN don't forget to bring up the bottom main drain.