I have a three story house with three hot water tanks we had to change the hot water tank in the upper story now everyone has hot water but the middle story of the house , the hot water was working fine before the tank was changed.

Are all of your tanks plumbed together? I have seen that before. While changing the tank on the upper floor, calcium deposits may have dislodged from the line/tank and blocked the second floor. You may have to flush (or back flush) the lines. I really have never liked the one-for-each-floor system. If your home is that big, either a central boiler should have been installed or a tankless heater for each floor. In the case of a central boiler, a recirculation pump is used to keep hot water circulating through the pipes for instant hot water.

Yes! Give us more details about your set up. Are the three water heaters feeding a common system or is each heater feeding a floor? Back to you, Tom