I have two full baths upstairs and one half bath downstairs. All of a sudden the one for the master bath toilet bowl has a low water level. And the last few days the other bathroom toilet bowls have followed. Is it a leak somewhere? I don't see any water anywhere.

Were it just one toilet I might agree that it's either a leak or the little fill tube came out of the overflow tube but two toilets with low bowl levels indicate that the water is being siphoned out of both bowls. Do you hear any noise from these toilets?
Sure sounds like you have a blocked vent. These vents must be snaked from the roof. Let me know if you need instructions or will you be calling in outside help. Good luck, Tom