My bad


.I was almost sure you know that, but had to say it.
Roots like air for the res change time.(if they don't dry out for 1h

)
It depend on how much of ro water you was using (check your ec/ppm) and you will see when just topping up is not enough and you need to change res completely. 3 hungry plants + plain water will lower your ec/ppm quickly what will lead to res change more frequent than 1 week.
When you will cleans/flush you don't need to add nothing to the res water as the plant will just drink and use the reserves they stored trough the grow cycle. And you can use tap water as well, but if you want to clean them faster and cleaner ro water is better for sure.
It will work but believe me, it is pain in the ars

to sack the water every time manually...... (I need to do it always when I get a huge monster in my cab that I'm unable to move out of bucket). And you will have huge ammount of roots when they will grow up, it will not be easy for you.(and dwc is not the easiest way by it self,why make it even more difficult) You are low on space there and you definitely will get res temp issues, that in combination with rest of slime in the res can lead to some issues (you know)