Put the one back in, place something such as an old brake pad(s) in between the pistons and work them all out a little. The pad(s) are to prevent one piston from coming all the way out releiving all pressure on the other pistons. May have to work them back and forth a few times to get them to loosen up. I'd avoid splitting the caliper if possible, they can leak upon reassembly.

Wear eye protection!

Good luck!!!

__________________
"Rudeness is a weak man's imitation of strength" - Eric Hoffer