We have similar set up. Ours is a little longer. However instead of a leg our designer used a 9" cabinet on the ends. The granite company said that it was enough support that we did not need a seam or additional support.

Below is ours started.

Chris with a 20" overhand on a 108" length (if I am reading correct) you will need more than 3 legs, you will need complete steel perimeter framing. Including modifcations to your cabintes for steel support. Post up a floor plan and elevations of you have them and others can chime in but you definitely want plenty of support as that top will be HEAVY, and just think of the damage were something to happen (people as well as your home).

Try the Original Granite Bracket. That is what we plan on using for our island. Our island will be 9 feet wide 42 inches deep. The overhang for seating is 15 inches. it only requires a notch out of the cabinet for the support to rest in. In our previous house we had corbels that I smashed my leg on several times so I didn't want that this time around.

https://originalgranitebracket.com/products/hidden-l-granite-bracket?variant=1604871619

Chris,

Your entire slab, on average will weigh around 650lbs with 108"x20" overhang section weighing around 300lbs assuming a 1 1/4" thick granite slab, more if using thicker granite. A 650lb slab needs solid support.

You can avoid using any legs at all if you use Federal Brace Brackets; you will need 5 17" brackets for support. Admittedly brackets are not invisible but fairly hidden and offer least amount of obstruction to stools and seating.

I hope there is steel tubing under that between the posts and cabinets, and intermediate supports as well. Because point load supports like that are insufficient.

There is steel under there...it's just hidden by the mitred edge (in the end that was the reason for choosing that edge profile. Hides everything nicely as you know).

Sophie so even brackets mentioned above would be insufficient along w the point supports?