Which is larger - 0.75or 0.750?

Neither. They are both the same number. The only difference would be from a scientific point of view. If you were to measure a substance, saying that you have 0.75 mL of it would mean that it is accurate to 1/100th mL. Saying you have 0.750 mL of the substance means that your measurement is accurate to 1/1000th of a mL.