"scripts": {
  "start": "parallelshell 'cd app && ionic serve' 'cd api && npm start' 'cd assets && npm start'",
}

...
"scripts": {
  "client": "cd client && npm start",
  "server": "cd server && npm start",
  "assets": "cd assets && ionic serve",
  "start": "concurrently "npm run client" "npm run server" "npm run assets" ",
},
...
"devDependencies": {
  "concurrently": "^1.0.0"
}
...