The group of swimmers was drifting along Jefferson River in inflatable ring tubes on Wednesday evening when they spotted one or two of the animals.
"An otter approached and attacked them," Montana's Fish Wildlife and Parks (FWP) service said in a statement. "All three women were injured."
FWP said otter attacks are rare but can be caused when the semi-aquatic animals are protecting their young, which take to the water in summer, or believe that a food source is under threat.
It advised people to keep their distance but, if attacked, to "fight back, get away and out of the water."
FWP official Morgan Jacobsen told a local NBC affiliate that the women had telephoned for emergency help and that one victim was flown to hospital due to the bites on her face and arms.
Several states away, surfers in California were the target of sea otter attacks last month when a five-year-old female tried to steal their boards.
In one social media video, the otter climbed on a surfer's board and hung on as he tried to shake it off. The otter lunged at him, and started biting the board.
Authorities in Montana have posted signs warning of the danger from otters, the FWP said, but are not planning any other response.
It added low water levels can bring humans closer to wildlife.
Otters, members of the weasel family, were historically sought for their pelts, with hunting and habitat loss threatening the population of both river and sea otters.
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