Here's a technique I have used, though depending on how high your boat sits off the ground when it's on the trailer, it may not (probably won't) satisfy your 3 ft off the ground requirement. This must be done on LEVEL, HARD ground.
Lower the trailer tongue jack all the way down. Put blocks under the chines at the rear of the boat. The last time I did this, I used two foot sections of wolmanized 6 x 6 for blocks. With the blocks under the chines, raise the tongue jack. The rear of the boat should now be off the trailer and on the blocks. Next, put blocks under the keel, towards the front of the boat, just behind the forward most cross member. Lower the tongue jack. The boat will now be off the trailer. But you got to get the trailer out from under the boat. Pull the trailer foward until the next cross member approaches the keel blocks. Put a floor jack behind the cross member, and jack the boat up so it's lifted off of the keel blocks. Remove the keel blocks and pull the trailer foward until the next cross member (or axle) approaches the floor jack. Put the keel blocks back under the boat in front of the floor jack, lower the boat onto the blocks, and re-position the jack just behind the cross member (or axle) Keep repeating this to get past all cross members and axles. It's a bit of a PITA, but you'll get there. I hope I described the process clearly. I can't emphasize enough that this must be done on flat, hard ground. Take your time and do this slowly and carefully. I've used this technique several times over the past 15 years or so. I have never done this with a boat greater than 22 ft however. I have also read this same technique being recommended in Trailer Boats magazine. It does have the potential to be extremely dangerous if not done properly. If you do not have an area of flat, hard ground to do this on, or you are not confident in your skills or abilities, I recommend that you look for an alternate approach. The last time I had to remove a boat from a trailer, I used the hoist at my Grady dealership. Then again, it was because of their inability to set up a trailer properly that I needed to do so.
