Lars,
Pterostylis curta at the very least is frost hardy to some degree. My shade house gets below zero by at least a few degrees celsius, and they thrive and flower in there without a problem at all. I don't specifically grow the other two species (I think the last one would likely be obtusa, not ontusa, just in case you have the wrong name on it) but I grow a bunch of Pterostylis and they are all in the same conditions. That said, they definitely don't like heavy frost directly onto them. My shadehouse offers some protection and they are at the top to minimise frost. They all take way lower than 8 to 10'C, so you may not need to keep them as warm as you are.
I hope this is a help.