I need to know if my employer in Maryland can refuse to allow me to use sick time I have earned if meet the requirements. I am told that from now on if I am out sick three days, I will only be paid for one, regardless of how many sick days I have earned and if I notify my supervisor prior to my shift, etc. This is a change from the previous policy and I have a lot of sick days so I don't abuse this.

Since your employer is not required to offer you paid sick time in the first place, they can pretty much put whatever rules they want into place about how it gets used.