I have an electrical heater with three separate elements, which each use 1133 Watts, so the entire appliance is rated at 3400 Watts, 240 Volts, 15 Amps.

I would like to reduce the output of the heater. Would it be safe to simply remove two of the elements, leaving one running, to turn it into a 1133 Watts heater?

Probably can be done but I personally would not modify a heater that was not designed to operate differently than when sold. Heaters cause many house fires. If you insist on doing it, call the manufacturer and get an opinion. I am betting though that the manual for the heater warns against modifications. Could an insurer refuse to cover a loss for which a rewired heater was determined to be the cause, I don't know. See what others here say. I tend to be overly cautious at this stage of my life.

I think it can be done, but I honestly don't see the point. If you're thinking it will use less electricity think again - sure the max power draw will be reduced, but the heater will have to run longer each time it kicks on, and so the total amount of electricity consumed (measured in kilowatt-hours) will remain about the same. Either that or you'll end up with water not fully warmed to the temperature you've set - but if you're OK with that then just turn down the temp on the heater without making this modification and you'll get the same result.