i have no red botton on my apple series 3 ? why ?

Hi Jeans,

There are two types of Apple Watch Series 3 model - one with GPS and one with both GPS + Cellular:

Apple Watch - Compare - Apple

The (GPS + Cellular) model has a red dot on the Digital Crown (the round button on the side):

Identify your Apple Watch - Apple Support

If you do not have a supported cellular service plan on your iPhone, then you will not be able to set up the cellular feature during setup, but it can be activated later via the Watch app if and when you do have a supported service plan (on your iPhone, in the Watch app, go to: My Watch > Mobile / Cellular Data):

• Apple Watch - Carriers - Apple
• Set up and use cellular on Apple Watch Series 3 (GPS + Cellular) - Apple Support

If you have an Apple Watch Series 3 (GPS + Cellular) model, then I suggest that you contact your carrier to ask about supported service plans.