The 3CX instance manager is one thing we often recommend (and use ourselves!) due to the ease at which you are able to monitor & manage multiple PBXs.
Enabling the instance manager is simple, and can be done directly from the 3CX Management Console in question. From here, you can assign permissions such as monitoring only, management, or full access to everything. This can also be extremely helpful to larger organisations with many locations, as your 3CX admin is able to manage the PBX from whatever location they are in.
Once the Instance Manager has been set up, each PBX managed can be updated, monitored, restarted and more – either all at once, one at a time, or specified by the administrator manually.
3CX offers a great guide on setting up your Instance Manager, found here.