Yes, you can have a DT92 and two HR92's in the same zone no problem.
First make sure your zone is in "single room" mode. If it is not then the HR92's would have each been measuring their own temperature locally rather that using one HR92 as the master temperature sensor for the zone.
When binding a zone from scratch first bind the device that is going to be the temperature sensor in this case the DT92, then you will be prompted for any additional "actuator" devices to bind - bind your HR92's one at a time when prompted.
By the way if you already had both your HR92's bound to the zone in single room mode all you needed to do to add the DT92 was to press the option to choose the sensor - choose remote, then bind to the DT92. This will leave the HR92's bound as actuators (to control the valves) but will transfer temperature sensing responsibility to the DT92.