Softex on a 3?

The answer is yes. However, likely the dealer will take the car to a custom upholstery place that sells leather and they'll put leather in the car. That's why they told you that. Toyota probably isn't making their soft-tex available to anyone.

Sure the dealer could take out seats from a Four and put it in a Three, but they'll NEVER do that. So as orenji says, yes, with money it's possible. Wait for a wrecked Prius Four to come available, buy all the soft-tex stuff from it, and then swap the parts.