Well first off, the Elements of water heaters are typically just Resistors, so they are "Linear" loads as far as power goes [almost all True Power / Pure Resistive load].
What's all this baloney mean you ask?? Don't ask me
Pretty sure that you will be able to use DC for the elements, as the RMS value of AC is equivalent to the heating effect of DC at the same levels [Voltage, Current and the resultant Power].
Problem is that you will need to have a voltage near 240 volts, in order to push enough Current through the element to equal the desired 3 Kilo Watts of power. If the voltage is too low, very little Current will be pushed through the Element - resulting in nothing more than a power consuming, meter spinning, water-warming [somewhat] deal!
Using Ohms law to figure the Element's Resistance [which might actually be the AC Impedance!], this is what comes up:
3000 Watts @ 240 Volts = 12.5 amps
12.5 Amps, pushed by 240 Volts = 19.2 Ohms Resistance.
Check the DC Resistance of the Element with an Ohmmeter, then figure what Voltage is needed to push a level of Current through it, which will equal 3000 watts.
The most simplest way to do this would be a DC to DC converter - which is usually an Inverter with a Rectifier for high power situations.
If you can connect your sources in series, that might work out...???
I'm not sure of what levels you have from your sources [Voltage, Current, Amp-Hours, etc.], or the possibilities available for connection schemes and/or storage vs generation, so fel free to throw some numbers in!
I'm sure that other members will be able to offer you much better answers or solutions than these!