Volume is not constant; it is directly related to the number of moles of the gas. (mols gas * 22.4 = volume gas @ STP) Temperature and pressure are constant.
Boyle's Law states that as the pressure of a gas increases as the volume decreases. There are several ways to state this relationship. P is proportional to 1/V. You might remember the shape of y=1/x...
A constant voume is an amount of space that does not change. An example would be a rigid box. No matter what you put in that box, it remains exactly the same size. So the volume is constant. Constant...