Is it safe to use my Apple Watch series 3 at the Sauna? The temperature at the Sauna is around 180 tp 200 Fahrenheit

Hi

Apple specifically advises that Apple Watch Series 3 should not be worn in a sauna:

• About Apple Watch water resistance - Apple Support
• Important handling information
• Keep Apple Watch within acceptable operating temperatures - Apple Support

Note also that:

• Apple Watch Series 3 models are sufficiently water resistant that they can be used, for example, when shallow water swimming, but they are not waterproof.
• Apple advises against exposing Apple Watch to soap or soapy water (such as when showering or bathing) or to high velocity water.
• Apple advises against subjecting Apple Watch to dramatic changes in temperature and/or humidity.
• Apple Watch should be kept within the recommended ambient temperature ranges.
• Leather and stainless steel bands are not water resistant.

Read here:

Keep Apple Watch within acceptable operating temperatures - Apple Support

The Apple Watch should not be operated in environments which exceed 100°F or are below 32° F.

As to Water Resistance vs Water Proof:

• Water-resistant: able to resist the penetration of water to some degree but not entirely
• Water-repellent: not easily penetrated by water, especially as a result of being treated for such a purpose with a surface coating
• Waterproof: completely impervious to water

http://www.hzo.com/waterresistant-waterrepellent-waterproof-whats-difference/

The Apple Watch is water resistant under certain circumstances. Certain models can be used while swimming or showering.