what were three important science contributions by the tang dynasty'

The famous astronomer, Monk Yixing, was the first to successfully measure the length of the meridian line.
The well-known King of Medicine, Sun Simiao, wrote a medical book Qianjin Fang (Thousand Golden Prescriptions) which was considered the treasure of traditional Chinese medicine.
In 868, the Chinese translation of the Diamond Sutra was printed, nowadays recognized as the earliest engraved printing in the world.