Is Amazfit GTS 3 water proof?

Yes. Amazfit GTS 3 has a water resistance grade of 5ATM. It can be used for water activities in shallow water such as in a swimming pool or at the beaches. However, it does NOT support sauna, hot springs and diving.
In accordance with GB/T 30106-2013 / ISO 22820:2010, a device that's water resistant at a depth of 50 meters shall be graded as 5 ATM.
I. This product is water resistant up to 50 meters (5ATM). It can be used when swimming in pools and shallow beaches. However, it can't be used for diving, water skiing, or other activities involving high-speed water flow or immersion in non-shallow water. Please also refrain from wearing the watch while taking hot baths or saunas;
II. Avoid immersing the watch in water for extended periods of time. Do not operate the watch under water. If the watch comes into contact with water, wipe it thoroughly with a clean soft cloth;
III. Avoid exposing the watch to any sudden temperature changes (such as using a hair dryer to blow the watch with hot air);
IV. This product is not resistant to corrosive liquids such as acidic or alkaline solutions and chemical reagents;
V. Water resistance is not permanent and may weaken over time. Do not re-check the watch for water resistance or re-seal it to resist water. Dropping or other impacts will also affect its water resistance. Any damage or defect caused by abuse or improper use are not covered by warranty.