There's a great thread on Garden Web about Japanese maple varieties growing in zone 5. If you do a search for "Zone 5a and 4b Hardy Japanese Maple Cultivars" you'll find it.

I can't see your landscape, but personally I would use three different shapes of JM rather than three mounding dissectums. Even many of the upright varieties only grow to 8-12 feet, so you wouldn't be planting a space-hogging monster. Perhaps one vase-shaped upright, one mounding dissectum, and different shape/color to finish out the triad? A dwarf globe, or one of the dwarfs that takes on a miniature tree shape? If you really like the dissectum leaf, you could choose Seriyu as your upright green.

I'm in zone 5b, in a sheltered planting area, and my dissectum varieties are Tamukeyama, Orangeola (which may be borderline), and Lemon Lime Lace. I also have a Seriyu. My favorite upright by far is Fireglow, but I'm also enjoying my bright Moonrise (a shirasawanum variety, which is said to be more hardy...you might also like the look of Autumn Moon). Iijima sunago is quickly becoming a favorite, too.

For knowing what will grow for you, try the "Maple app" on the Davidsans web page. You can put in color, leaf shape, size, and temperature, and it will generate possibilities for you.

Good luck!

Thank you for the GREAT ideas!