server:run Exception There are no commands defined in the "server" namespace

As I mentioned in my comment, in S3.3 the server commands have been moved to their own WebServerBundle. Some editing of AppKernel.php is required to activate the bundle. I suspect that many other developers might run into this once 3.3 is actually released and people try upgrading.

I make no promises but try updating AppKernel.php with:

    if (in_array($this->getEnvironment(), ['dev', 'test'], true)) {
        $bundles[] = new Symfony\Bundle\DebugBundle\DebugBundle();
        $bundles[] = new Symfony\Bundle\WebProfilerBundle\WebProfilerBundle();
        $bundles[] = new Sensio\Bundle\DistributionBundle\SensioDistributionBundle();

        if ('dev' === $this->getEnvironment()) {
            $bundles[] = new Sensio\Bundle\GeneratorBundle\SensioGeneratorBundle();
            $bundles[] = new Symfony\Bundle\WebServerBundle\WebServerBundle();

I don't have a good test project to try it on but at least the commands should show up.

And in case anyone is wondering, all I did was to install a fresh development project and poked around a bit.

composer create-project symfony/framework-standard-edition s33 "3.3.*" --stability=dev

I also found this but it does not mention the need to update AppKernel.php

From the upgrade guide:

The server:run, server:start, server:stop and server:status console commands have been moved to a dedicated bundle. Require symfony/web-server-bundle in your composer.json and register Symfony\Bundle\WebServerBundle\WebServerBundle in your AppKernel to use them.

As long as your composer.json has symfony/symfony in it then there is no need to add the web server bundle to it. Just need to adjust the AppKernel file.

run this command before starting the server.

composer require symfony/web-server-bundle --dev

you must execute this command:

composer req webserver