Installed Electric Tankless Water Heater - Internet loss when active
First, make sure that your three supply cables are paired properly.  The unit has 3 heaters internally.  There are several ways cables could be crossed that would result in power from 1 heater coming up one cable and returning on another cable.  The heater would work properly, but would kick stupid large amounts of EMF from the two imbalanced cables.  Just double-check it. 



Or, the heater may be throwing high frequency electromagnetic hash, with the cable(s) becoming antennas and making this worse.  Now, electric resistive heating elements, alone, do not throw electromagnetic hash.  That would only happen from 


a) control electronics, which will surely only be on one cable/circuit; or 
b) power choppers ("dimmers") which could be on 1 or all of the circuits, but there'd be no useful reason to have them on more than 1 circuit.  


Also, there's a thing I don't like about how Ecosmarts are installed - they expect you to cram three #8 cables through one 1" hole, which is a lot to ask, and requires just the right cable clamps to not violate Code.

I would install a 6" square deep or larger metal box right below the EcoSmart and a short EMT conduit nipple connecting it to the EcoSmart's wiring hole.  Bring the 3 cables into the large box with correct clamps; leaving space for up to 3 "surge suppressors" to be fit onto 3 knockouts.  

Then I would consult with EcoSmart and ask them a) which circuit powers the control electronics and b) whether they merely switch all 3 heaters, or whether they use power choppers, and if they do, on which circuit(s).  

On the circuits which do not use choppers or electronics, those can be wired straight through the 6" box, up the conduit nipple and to the terminals.  

For the circuit(s) which do use choppers or electronics, you run a pigtail from the EcoSmart's terminal into this box.  Then you mount a "surge suppressor" in one of the knockouts (or not, if it's a type that can live inside the junction box e.g. Meanwell SPD-20-277P). Then join those wires to the power line from the panel. This wires the Ecosmart and the suppressor in parallel (the usual way you "tee" a connection) - it's not wired in series like a switch. 



At this point, the surge suppressor(s) should be damping out the EM interference before it leaves the metal faraday cage of the Ecosmart + junction box. 

I suppose Ecosmart could build this into the product, but that would raise the price-point, and so you might not have selected the product.