Manual Plugin installation on Craft 3
You have to create the folder where you want to include your plugins by yourself. There are no requirements how you name it. Let's say you want to create the folder plugins in your Craft folder, then you have to include the plugin in your composer.json like this

"require": {
   "php": "&gt;=7.0.0",
   ....
   "namespace/pluginhandle": "your version",
   "secondred/form-builder": "^1.0" (&lt;--- example how it could look like)
},
"repositories": [
{
  "type": "path",
  "url": "plugins/yourPluginFolderName"
},
{
  "type": "path",   
  "url": "plugins/form-builder" (&lt;--- example how it could look like)
},
....


Then you'll need access to your server via ssh and do composer update