You need to remove the front wheels and proceed to squeeze back each caliper piston - but first lay a rag over the brake fluid reservoir cap vent to avoid squirting excess brake fluid. You could use a squeeze bulb to remove some fluid first. Each piston should be able to be squeezed (with large channelock pliers or a clamp) or pried back smoothly. If they will not go back open the bleeder screw. If they go back with the bleeder screw open you have an internal problem with the flex hose. There may be one or more of the 3 pistons actually stuck. I would use brute force to pry them back--or remove a pad and then slip a thinner than the pad block in its place so the piston can't extend too far--then use the brake pedal to pump the piston outward to get it moving.
If you get the pistons to move in and out bleed the system to remove contaminated fluid and then road test for even braking. You may need to plan on caliper replacement or rebuilding. -- Dave