"Mr. Ebert may be a good movie reviewer, but knows little about physics. A tripod is a very stable platform. a four legged platform always has the tendency to wobble about on 3 legs unless the surface and the legs are perfect level and a perfectly length. A tripod always has all legs on the ground."
A tripod with rigid fixed length legs is stable in the static condition as in the case of a three-legged stool, but not necessarily in the dynamic condition as in locomotion.
When analyzing three versus four (or more) legs for stability, it is necessary to differentiate between the static and dynamic conditions, and whether the legs are of fixed length and rigid geometric configuration as with a stool, or effectively of variable length and geometric configuration through the use of joints.
With only three legs, at least one leg must be off the ground at a time to move, leaving at most two legs in contact with the ground. This leads to instability unless there is some effective balancing mechanism (i.e., sensors, a means of balancing by shifting the weight distribution, and a control loop).
In nature, legs come in pairs or even numbers, due to bilateral symmetry.