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The only way I see it being close to right: Tie both natural draft flues(water heater 3") together with a 3x3x7" tee, run it past your flues for the induced draft furnaces and tie it to your 8" connector past the connection for the furnaces. At the furnaces tie them together with a 4"x4"x8" tee, and run the 8" to the connector going through the roof, that would be a 8x8x7" tee, the 7" going back to your water heaters...the only thing I would worry about there is a possible back-draft while the furnaces are running. Simply put, you are trying to vent too much out of too small a hole.That seems the most gracefull, and closest to code fix I can come up with. Good-Luck.
You love giving FREE quotes, I know you do.
There's not much time involved is there?