Apple Watch 3. To charge it, what do you plug it into??

You should have received a charging disk and a little charging brick

Plug the charging disk cable into the charging brick and the charging brick into the wall.

The Charging brick should have the electrical plugs compatible for your country. What I show above is the U.S. plug style.

You can also plug the charging disk into a computer USB port.

There are other USB power source devices you can get (or might have) that you can plug your Apple Watch charging cable into.

The charger it came with which is magnetic. There is no port on the watch to plug it into. If for some reason you did not get a charger. Then order a replacement from Apple or there are many alternatives, try Amazon.

I just check the Apple web site, clicking "Buy" on each of the models, and

The Stainless Steel seems to have the power brick.

The Nike seems to have the power brick.

The Hermès seems to have the power brick.

The Titanium seems to have the power brick.

The Ceramic seems to have the power brick.

But did the OP say they had any of those models. They apparently just bought an Apple Watch 3 which means all those other models you listed are irrelevant and I didn't mention them nor did the OP. Another senior member here frequently answers the question about the power adapter buy posting a graphic with "What's in the box" and points out they need to read the fine print where it says that only the GPS+cellular aluminum models come with the power adapter.

If you go onto the site and look up the Apple Watch 3, click on buy and scroll down to What's in the Box under the 5w power adapter is says this: "5W USB Power Adapter

(GPS + Cellular model only)"

I didn't make it up.

The only get the power adapter if they bought the GPS+cellular for the aluminum model.