Is that a 4" draft hood on WH#1? Anyway, manifold the two WHs together with as much vent rise straight up off the draft hoods first. Here's how you do it:
Run the B-vent (sized as a common vent for all three) into the basement according to the listed instructions. Remove that fire hazard insulation. Where is this B-vent going and how is it properly supported with that 90ell over the foundation wall? I doubt it is properly firestopped or supported with clearance.
If you use B-vent as a vent connector, you can run the connectors within an inch of the ceiling (but add a little for fudge factor). Otherwise, at least 6 inches below the ceiling. You can connect to the common vent two ways: one common manifold or WH manifold in above the furnace. Since the furnace is a CAt. I fan assisted 80% furnace, it too needs all the lift it can get. the run all vents 1/4"/ ft downhill. Need better support, too.

I would use one common manifold but..........when you connect draft hood equipped appliances to the furnace, you defect the primary safety controls ( vacuum switch and spill switches). You can have a 100% blocked B-vent up top and it will vent right out the WH draft hoods without tripping the safeties. Just did one last week. I'll post pics another time. What to do? Install spill switches on the WH draft hoods connected to the WH thermocouples. When they get tired of re-lighting the pilots, maybe they will change those two WHs for one power vented one out the side of the home.

This is a bad setup no matter how you vent it.

Bob

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Keep the fire inside the fireplace!
IAFCI Regional Director & Certified Fireplace Inspector
Certified Master Hearth Professional & HPBA BoD