My son has a 3 wheeler much like yours. Yes, they are extremely unstable, with a high center of gravity. Letting air out won't help, IMHO. Be careful on turns, as they will flip easily.
There are trikes with much lower centers of gravity, and 2 wheels in front, and there are 4 wheelers. These are generally expensive, but possibly you might find one on Craigslist.
I believe there is a person on this forum who uses one regularly. Perhaps that person will chime in with ideas.