Hi! To better understand you, you have looked at your devices here:
account.microsoft.com/devices
But you cant remove the third pc from it and you don't see your other 2 PCs, if that's correct
To remove the device, click on the see details options of the device you want to remove, then on the top right corner you will see the option to unlink that device.
In regards to link devices, this is something that happens automatically, only Microsoft manufactured devices can be added manually, you only need to log your account as the main account of that device and it should connect to your account, there's no way to force this.