I believe you're going to find it won't work very well like that. If your house is kept very warm it might. Otherwise I think you're going to see that the first heater will stay on nearly all the time. The second one maybe off and on, or it may be on all the time and the third one off and on. I don't think I have 350 gallons of water and Iam using four heaters.3 heaters setup to go in three stages. On my new build (about 350G total) it will be
stage one 300W
stage two 200W
stage three 300W
To further add redundancies, the first two can be plugged via RANCO controllers
Additionally, they should be plugged into a different power strips on your controller (if you have) running off separate circuits.
Hope this helps.