First step, after all the bolts are out, is to hose them down real heavy with penetrating oil, then let them sit a day.
After that, I use something like a chisel or wedge driven into the bottom of the tree to spread it open slightly. You have to be careful to not hit the fork leg. If they will move any, give them a good spin, one way then the other while pulling them out.
Before putting them back in, use a wire brush on a drill to get in there and thoroughly clean the tree. Then smear a light coat of copper (not the silver stuff) anti-seize inside the tree and on the fork legs. Next time, they should slide right out with a good tug.
If they've been in there since the last ice age, back when Woolly Mammoths in cut-off jeans rode trikes, it's going to take some time and patience to prevent damage.
The story of three wheels and a man...