Magnatiles!!! My son had these at his preschool and we loved them so much we got a set and have also bought them for my nephew and nieces. They are flat acrylic tiles with magnetic strips inside so you can connect them. They teach geometry while kids play and best of all, they can be used by kids of different age groups. We got my son's when he was about 4- he is almost 10 and still uses them.
Sometimes he just builds buildings with them, they have made Matchbox car garages, they have been forts for action figures and Transformers to break apart, they are never-endingly creative. There are several different packs- some even come with wheels, etc. but the basic shapes are just fine.
They are expensive, but ALL three kids can use them together or separately and you will get YEARS of play out them. When my son is too old to use them, they are one of the toys going into the 'grandchildren' box (along with the Brio trains) for when he has kids of his own.
If you are in the Chicago area- go to Geppetto's Toy Box in Oak Park on Lake Street. They have them there, and also specialize in the kind of long-lasting, battery free toys it sounds like you want. The staff is super nice and helpful and they have a lot of really great toys you won't be able to find at a Toys R Us. Also, they are a small, family-run toy store! I think they do online orders as well. Their website is: www.geppettostoybox.com