Been there and done that. All you need is about ten or fifteen feet of stout tow chain with hooks on both ends.
1. Back the tractor up to the post so the post is between the lift arms.
2. Wrap the chain around the base of the post several times.
3. Bring the ends of the chain up and over the lift arms and link the chain back to itself. Try to keep the everything as close to the PTO as possible to get the best leverage.
4. IMPORTANT: Inch forward until the chain becomes tight and all the slack is drawn out. If you wrapped the chain tightly you should only have to move forward a few inches.
5. Turn up the throttle and lift the three point as slowly as possible. You'll feel the post inching up out of the ground. Once you break the bond between the concrete and the ground the post gets easier to lift.
6. When the three point is at it's vertical maximum you'll only have lifted the post two or three inches. Lower the three point, loosen the chain and slide it back down the post and repeat the process. Eventually the post will be loose enough to take the chain off the lift arms and hook it to the bucket to lift up the post. SAFETY NOTE: Only use the bucket to lift a "free" post. You can easily tip the tractor attempting to pull a "stuck" post with the bucket. The leverage just isn't there to do the job.
7. Make sure your helper is well out of the way when you gently lower the post after removing it from the hole.
8. Stand over the empty hole with your helper and contemplate how deep it is.
NOTE: I've found it's fastest to have two chains and two helpers. Each helper is responsible for wrapping the next post and hooking the chains over the lift arms. I've used this technique for wood posts in concrete, wood posts set in the dirt, and old stuck tee-posts.
