How the time is converted into distance by multiplying it with 17,000
If you take 340 m/sec (approximate speed of sound through air) and convert to cm/sec you get 34000 cm/sec. For pulse-echo, the sound travels twice the measured distance so you need to divide the conversion factor by 2 so you get 17000 cm/sec. When you multiply by the measured time, you get distance from the transducer to the object in cm.

The other two conversions are converting from time measured in microseconds at the same time so the formua for Distance in centimeters is the same as: Distance (cm) = Time (seconds) * 1000000 (microseconds per second) / 58 which comes out to (approximately) Distance (cm) = Time (seconds) * 17241 which is nearly the same as the formula in your question.

As Andy said, the speeds of sound used in the formulas are approximations. The actual speed of sound through air varies with temperature and (to a lesser extent) with humidity (and a little due to other factors).