With one adult and two 70 lb kids it porpises (bounces up and down) badly over about 20 mph (guessing, no speedometer). I tried sitting in front and in back of the kids and it did not make a difference. Can I install trim tabs on this thing to fix this?
The waverunner III was never a real stable ski. I can tell you what works for some skis is a new intake grate (keeps it hooked up in the bouncy stuff), a new ride plate (extends to provide more stability), and new sponsons (which will help turning stability).
This may be more that you want to invest in an old ski, but there is not a whole lot else you could try.
Never heard of trim tabs on a ski, but might help. You might want to try some Smart Tabs. They are self adjusting. I've got some on my boat and they work well to get it on plane faster, but I wouldn't say they do much to improve stability.