you don't need to press before adding on the 3rd diamond.
just follow the video exactly and you'll have all the perfect cubes you need in about 60 minutes.
our church made a queen size tumbling block to sell and myself and 1 other person did all the cubes in 75 minutes.
we didn't press anything until the cube was done. we had 1 other person taking the finished cubes off the back of our machines for pressing.