As long as the rhp are all identical you can use one tstat. That is as long as the three rhp do not exceed the power load of the single plug. Three should be fine, I have three set up like that.
If you think you're going to expand a Herpstat 4 or 6 may be a good choice. I have two 1s, a 2, and a 4. They have all worked great for me.
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