I currently have two line voltage ceiling radiant heating systems in my condo each with their own DPST thermostat. I'd like to use low voltage thermostats such as the nest. I found this transformer/relay, but it only has 3 wires. Will this work with a system designed to use DPST thermostats? If not, is there something else out there that will allow me to use low voltage thermostats with line voltage heaters?
The device you linked to will work. You are essentially converting to a SPST "thermostat" from your current DPST thermostat. Some people feel a SPST thermostat is dangerous because the heating device is always connected to a hot leg of the power mains. Both legs are disconnected with a DPST device. Yet this SPST practice is common and acceptable in many jurisdictions. It's quite safe, there is no way to contact energized components in a proper installation. And when the heat is on, all components are energized under either system, and yet it's still safe.
The only conceivable danger is if someone assumes it is a DPST system and foolishly attempts to work on the heater without turning off the power at the breakers. If you're OK with this marginal increase in risk, check with your local building authority, ask them if you can use a single pole line thermostat on a 240 volt (assuming US/Canada voltage system) resistive heater. The remaining details aren't pertinent.
If all is OK, the device's installation instructions are fairly clear on how to hook up both 2 and 3 wire low voltage thermostats. Just ensure you order the correct device for your heater's voltage and power or current rating. To be safe, the device rating should be 125% more than your heater's actual power or current draw.
If you or your building authority do not like SPST type installations, you should be able to find a similar relay that is DPST, even if you have to supply the transformer separately. Any possible solution will certainly involve some combination of transformer and relay, it's just a question of packaging and proper ratings.