Hi
The cost of AppleCare+ for Apple Watch varies by Series 3 model.
The cost is shown when adding your preferred model to your bag in the Apple Online Store. In the US, for example:
Buy Apple Watch Series 3 - Apple
Information about AppleCare+ for Apple Watch is available here (for the US, for example):
AppleCare+ - Apple Watch - Apple
The page includes a Buy Now link for online purchases and information (at the bottom of the page) for buying via an Apple Retail Store or by phone.