Go to node_modules/@vue/cli-service/lib/options.js
At the bottom inside the "devServer" unblock the codes
Now give your desired port number in the "port" :)
devServer: {
open: process.platform === 'darwin',
host: '0.0.0.0',
port: 3000, // default port 8080
https: false,
hotOnly: false,
proxy: null, // string | Object
before: app => {}
}