Hi Tangentspur,
A few books that might have ideas to help:
- Early Gifts: Recognizing and Nurturing Children's Talents
- Your Gifted Child

Both of these books are about responding to giftedness in young children and both include some interesting ideas about how to meet your child's needs outside of a school environment.

Also, Teach Me To Do It Myself: Montessori Activities for You and Your Child, while not specific to giftedness, has some different approaches for a range of subject areas.

If he's into cars, have you gotten him thinking about engines? There's a Magic School Bus episode on how an engine works, and there are probably some good nonfiction books for kids on basic mechanics/engineering of cars and "things that go."

Have you tried puzzles? My 3 yr old girl loves 50-100 piece puzzles and can do them mostly by herself. The Melissa and Doug floor puzzles are pretty good -- maybe there's a car-themed puzzle? Puzzles promote fine motor skills, spatial thinking, a whole range of stuff.

Other things you can do patterning with: legos or unifix cubes, a pegboard, large beads on a pipe cleaner, or pattern blocks (we like Mindware Imaginets, although there are other good options that are less expensive).

I'm not much of a social media person, myself, but I have found Pinterest to be useful for finding good ideas that don't have to cost a fortune -- maybe it's worth a look?

I love how you incorporated the matchbox cars into getting him to write! That's really creative and exactly the kind of thing we have to do with our kids.

I can hear how tired and somewhat discouraged you are, and I think you're doing amazing! I only have one, and I can't imagine managing three on a daily basis. Just the fact that you're doing everything you can do puts your kids in so much better of a position. Hang in there!