Tokyo Electric Power (TEPCO) said the men were building a 13-metre-high (43-foot) tank that was intended to store water used to cool broken reactors.
One of the three was knocked temporarily unconscious, a TEPCO spokesman said, and remained in a critical condition in hospital, where he had been taken by helicopter.
Another man appeared to have broken his leg, while the third was able to walk unassisted.
TEPCO has been rushing to make hundreds of tanks to store a huge volume of contaminated water until it can be cleaned and released into the sea.
Fukushima was hit by a tsunami in March 2011 which sparked meltdowns that spread toxins over a large area, forcing tens of thousands of people from their homes.
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