Chimney with furnace and wood stove? more info?

Ok, so heres the info, I bought a house, the gas furnace is vented through a large lined chimney along with a wood burner. The chimney has 3 thimbles, the previous owner said it used to have a gas furnace water heater and a wood stove, I haven't used the stove yet and have switched the water heater over to electric when it failed and closed off the thimble for this, my question is can I use the stove? If not what can I do to use the stove? I looked into getting a sep chimney for the stove and its rather pricey. Could I vent the furnace out the side of the house? do I need to get a stack for the Furnace? Any ideas?