The importmap in your code is essentially setting up a shortcut from the string "three" to the actual .js file URL. What you should be writing in your <script type="module"> is import * as three from "three"; and it will automatically resolve to the URL due to the importmap you defined before.

From https://github.com/WICG/import-maps:

By supplying the browser with the following import map

<script type="importmap">
{
  "imports": {
    "moment": "/node_modules/moment/src/moment.js",
    "lodash": "/node_modules/lodash-es/lodash.js"
  }
}
</script>

the above would act as if you had written

import moment from "/node_modules/moment/src/moment.js";
import { partition } from "/node_modules/lodash-es/lodash.js";