Three years old is a time to learn on his own. The pincer mechanism is very important for writing and being able to hold a pen or pencil when he is 4. Taking large tweezers and being able to pick up dry beans and peas is something to be practiced. He can take them from one colored tray/bowl and place them in another.
With summer fun activities, he will love to pour water from different containers into other unusual contaniers. This is where the sand box also comes in.... Sandscript alphabet cards is something that you might want to try with him. Remember, engage as many of the senses as possible and make up your game.
Three year olds who have been reading books, flash cards, advertisements, etc. will enjoy learning games, putting the words on one-side and the pictures on the other side (columns), it might be phrases, or even complete sentences, or even foreign languages.
Because children at that age like to touch things, you can make each side of the game board clear plastic pockets, using one color electric wire tape for one column and a different color electric tape for the other column. That way, you can just make sure that the flash cards fit. I made flash cards for colors, birds, animals, sea creatures, cars, planes, etc. It offered a wealth of enjoyment and wonderful pasttime as the older children would play the game with the younger children, if I needed to prepare dinner or whatever.