First, let me state that I have never set up a H-D sidecar with 3-point mounting; I sure there are probably folks reading this forum who have. However, the basic principles should apply.
That said, I have been running sidecar rigs on /2 Beemers for 30 years and presently have a Russian Ural military-style sidecar on my 2002 Victory V92C. My wife and I rode it from West Texas to San Diego and up the coast to Seattle and back a couple of summers ago. I have also set 4-point mounted side cars up on a variety of bikes for people when I owned a bike shop. Sidecar outfits handle weirdly but predictably when set up right. They have to be properly aligned, just like the front end on your car, if they are to handle well.
Park the outfit on a level cement slab and get a long straight-edge, say a straight piece of angle iron, long enough to make full contact along the sides of both motorcycle wheels. Draw a chalk line on the floor along this straight edge with the straight edge flush against both front and rear wheels on the sidcar side. Draw another line perpendicular to the straight edge in line with the bike's front and rear axles. The sidecar axle should be positioned 6 to 10 inches ahead of the motorcycle rear axle.
Next, place the straight edge along the side of the sidecar tire and draw a chalk line forward until it is even with the motorcycle's front axle. Measure the distance between the two chalk lines at the motorcycle rear axle to the new line. Then measure between the lines at the motorcycle's front axle. There should be 1/2 to 1 inch of toe-in, that is the front axle line should be 1/2 to 1 inch shorter than the rear.
Finally set the lean-out between the motorcycle and sidecar by adjusting the top mount or mounts. This is kind of trial and error. A solid-mounted sidecar that does not have provisions for adjusting lean-out on the fly is set up at something of a compromise; you want it to track straight at the speed you ride most of the time. Just keep fiddling with the lean-out until it tracks straight at 60 mph on a level road, if that is your usual cruising speed. Above or below that ideal speed it will pull slightly to one side or the other. A high crown on the road or a crosswind will also cause it to pull.
That's what I mean about it handling weirdly. You can never just take your hands off the bars while running down the road, the rig will always pull slightly to one side or the other. With wide bars and a correctly aligned rig the pull will be slight and easily compensated for. As Cotton said, a tight steering damper is a big help. The front forks may tend to waggle some at low speeds but should stop as speed increases. Weight in the sidecar helps with stability. A heavy steel-bodied sidecar like the Ural does not usually need ballast, but some of the fiberglass and plastic-bodied cars are tippy without a passenger and may need a sandbag or some bags of lead shot. It is possible to flip a properly set up rig, but you have to work at it. I think they are a lot more stable than a trike.
Because the drive wheel is off-center and you have all of the weight and mass of the sidecar off to one side, the outfit will pull to the right, with a right-hand mounted sidecar, when accelerating, and to the left when slowing. This is very predictable and can be used, with experience, to set the rig up for turns. When turning right, blip the throttle as you enter the turn and the rig will whip around to the right. Turning left, cut the throttle and tap the brake and the rig will wheel left.
I love sidecars. They are kind of Weird-Alice and not for everyone, but they are great for hauling parts, groceries, kids , and dogs. On a sidecar rig dirt roads, mud, or ice hold no fear. When my stepson was 9 his mother and I took him to Disneyland from Texas with all three of us on the sidecar rig. Really turned heads on Sunset Blvd! When you show up on a sidecar rig, folks act like the circus just came to town; you almost always get a smile. Once we pulled up at a motel in Santa Fe with an old Beemer outfit. The 60ish German lady at the desk was staring out the window as we came in. "Ach!" She said, "It reminds me uff der var!"
Hope this helps --- Randall