I can't help but wonder how many people you have seen with 3 legs. I have never seen or heard of that happening, so I believe it to be impossible.
no, go see a vet.
Actually, yes they can, depending if they are still in their nymph stage. A young stick insect will grow back a lost leg gradually each time it moults - the leg growing back to normal size with each moult. If an adult looses a leg then no, it cannot grow it back.
Yes, African Americans have an extra bone in their leg.
Letting it grow.
No. Yawning does not cause leg hair to grow. What does cause leg hair to grow (either thicker or darker) is shaving, genetics, and your overall hormones in your body.
Yes a brittle star loses its leg and takes about a year to grow back
If a spiders leg comes off it will heal and grow a new leg.
Yes. If a tarantula 'loses' a leg (perhaps to a predator) - over successive moults, the leg can grow back.
He has an extra tendon in his leg
No, he did not. No one can "grow an extra finger." Some people are born with extra digits, though. Beethoven, however, was not born with extra digits.
Cricket.
Nothing, The leg will soon grow back.