Or use the [APP] < group > (2.4.2 - Stable) to create one sensor. In this app you can choose to pick average (most logical for temp), min or max for each value.
Beware that humidity is a value that is temperature related. If the temperature goes up, the (relative) humidity goes down. If you use any calculation on it, I would have it dependent on your goal. In this case I’d probably use max for humidity. This probably also originates from the coldest room. I would then use min for the temp, which would probably give you a device that shows you the coldest room with the hightst humidity.
Make sure you don’t combine rooms that may throw the value off too much. If you combine both inside and outside sensors, the outcome will probably be of little use. If you add in the bathroom something similar applies, as when bathing/showering the humidity goes up a lot, even though the bathroom is probably the hottest room at the time.
Most people compare the bathroom humidity with the humidity of a room nearby to detect showering, and then turn the ventilation high.
Also note you should never turn ventilation off completely, regardless what the humidity is. On a 3 settings ventilation system, level 2 is required if people are present, 1 is only ok if no one is there for a longer period, off is never ok. So basically you pick either 2 or 3 if you are at home.