If you installed Norton 360 onto 3 PCs, and you want to install it onto additional computer, but you’re unable to uninstall it first from the old one, you should use Norton Management (lets you manage all of your Norton products and Norton product keys from one location). You can access Norton Management from within your Norton Account, under Norton Apps. From there, you can add and remove your devices, and even remotely install and manage your Norton products on the device.
If, for any reason, you have trouble using Norton Management, you can always contact Symantec’s customer service and ask them to adjust the licenses in your Norton Account. One of the best ways to contact Symantec is to use chat option which is free.