Why are there no three limbed animals?

No it isn't 4 limbs, or paired limbs has the advantage of being easy to do, and has the advantage of redundancy, which would lead evolution to select for it.

 

I don't know of any animals that have six, but plenty have four. Bilateral symmetry applies at the level of the organism, not each subdivision. We only have one eye and one ear on each side, for example. And even with bilateral symmetry, it does not apply universally. We only have one stomach, one heart, one liver. Humans have one uterus, although in many other animals who routinely bear multiple young it is divided into two sections.

No. That fin is not articulated. It's not a limb, with bones, muscles, tendons, ligaments, etc. Pure cartilage, IIRC.

A tripod may be efficient but it is notoriously unreliable. Three-wheeled vehicles are the most unstable configuration of all, and no one even manufactures three-wheeled off-road bikes anymore for that reason. I notice that three-wheeled highway bikes have come into vogue, but just wait until the accident statistics start to roll in. They don't have the solid footprint of a four-wheeled vehicle so they tip over more easily, yet they also don't have the ability to lean into a turn like a two-wheeled vehicle.

Ever try sitting on a three-legged stool? You won't do it twice! Office chairs nowadays have five legs!

There are actually six Kingdoms of lifeforms:

• Animals
• Plants
• Fungi
• True algae
• Archaea
• Bacteria

The archaea are single-celled organisms with no cell nucleus that somewhat resemble bacteria, but there are enough major differences to put them in their own Kingdom. They are not even as closely related to bacteria as the other four Kingdoms (the Prokaryotes) are to each other. Since they are tiny and live in places that are difficult to access, they haven't been studied as thoroughly as the other living things.

But not all plants have it, just as all animals do not.

Attenborough is a master at making biology both interesting and understandable.