Before I take them to the dump, is there anything I can do to restore them? Are there any chemicals to clean them? I was going to repaint them using spray paint but it was taking too long and almost a full can of paint and the chair (not pictured) still looks like it will need a second hand.
Usually those chairs take obvious UV damage from the sun. This can weaken them. (it does the same to child car seats, one legitimate reason to throw old ones out). If that's not a problem, the chairs are probably saveable...
...but you need to learn to paint. One rattle-can should do all 3 chairs. You went through a whole can because you're misusing it as a magical fixer, cleaner and surface preparer... And that does not work.
I would start by working each chair over with sandpaper, maybe 120 grit, to smooth off any oxidation and give a texture primer can bite into.
Then, well gosh, what do you have around and how much do you want to spend? Myself I do enough woodworking to have West System epoxy on hand, it's not a paint but it's perfect for stabilizing a stressed surface like this. I would use either that or a 2-part epoxy primer (or garage floor paint) and paint that on thin, with a throwaway chip brush. It would probably be about $5 worth of West System to do those. I like epoxy because I know it'll help stabilize and strenghen the chair. If I didn't have that, then maybe an oil based primer like Rustoleum white. I don't think a latex primer would stick, and rattle-can paint won't stick to it.
Then after the epoxy cures good and plenty (do not stack them for a few days!), I would sand a second time probably with a Scotchbrite pad, just to give the surface some texture paint can bite into. The surface should now be very uniform and hard and take paint well.
Then rattle-can it.