i have three SHT21. i want connect all on arduino UNO R3. i want library and please explain to me.
The SHT21 has a fixed I2C address so connecting more than one of them to a single I2C bus is not straight-forward but possible if really needed.
Why do you want to connect three humidity sensors to one I2C bus? The bus length should not exceed 0.5m so three sensors in that range don't make sense.
To connect an SHT21 to the UNO you should use a level converter because the device is operating at 3V3 and the I/O pins should not get more voltage than the Vcc pin is getting.
There are more than one SHT2x libraries on the Net for the Arduino platform.