a mix of bleach and water or dish detergent and water

mr clean erasers. they clean everything.

In secessive order:

Dish soap and a plastic scrubber.
Try a CLR type cleaner.
Pressure washer.

metho?
warm oil and metho

:)

Go to the auto parts store and buy some engine degreaser. Follow the instructions. Simple Green work well to. Sold at Sam's Club.

i'd try the car wash first. you know where you put in quarters and spray the car yourself

purple power not deluted will clean it

bleach and water and elbow grease

I have used all purpose bleach spray, or bleach water (Strong) to kill any mold, or try Tilex mold and mildew. Spray generously, and let stand, respray,and scrub. Whatever does not come of with the bleach, use a mr. clean magic eraser.
good luck!
oh, and simple green also does a good job for a preliminary scrub/soak.

after 3 yrs outside there's no bringing them back. paint them with Krylon Fusion spray paint. it's made to stick to plastic and comes in a variety of colors. they'll look like new.

That's not weather build up. That's the resin degrading. Nothing will fix it, but Krylon gives you new appearing chairs.