1) The probability is 0, because if 3 letters are matched correctly then so is the remaining one
2) The three points lie on a unique plane if they don’t form a straight line. The only way they would form a straight line is if the centre of mass is on an edge, which isn’t the case for a tetrahedron of uniform mass.
3) I can think of three ways, but all of them involve something more than a thermometer.
4) There are no more examples. Every three consecutive odd numbers contains a multiple of 3. Three even numbers are obviously not all prime.
5) Why should it not be able to return to its starting point? Do you mean that it gets too tired after walking 4 kilometers?
6) What if you take three pills? Is that dangerous?