Will one SQL instance work whilst the other 2 are patched? My change request to patch 3 SQL instances on the same server was denied because of one instance - they say a server reboot is fine, but not a few hours downtime. 

Seeing as each instance is standalone, should it not be fine to patch 2 instances and leave the third? 

I see there is a 'shared components' feature - what is this exactly, is this down for all instances during the patching, and does it matter?

How can I assuage the fears of my colleagues that the third instance will still be working - the business are not willing for it to go down even for a couple of hours in the evening.
Firstly, it always better to test patches in DEV environment before applying on Production. 

To answer, there won't be downtime/interruption for Instance 3 while patching/removing path on Instance 1, all you need select particular instance during the wizard or command line as follows:

KBXXXX.exe /qs /IAcceptSQLServerLicenseTerms /Action=Patch /InstanceName=Intance1


There might be restart required (manual restart) to proceed with Instance 2 patching after completing instance 1.

Shared features are updated (patched) once. In your case during the Instance 1 or which ever starts first, and not applied while patching other instances on the same server. We never experienced any interruption with shared features (considered following): But if you got Integration Services and it's heavily used you must find maintenance windows where it's got less overhead. 


Client Tools Connectivity
Client Tools Backward Compatibility 


My environment was 3 instances on 1 server, all of them are AG replicas. One primary other two secondary, and there are 3 servers total with same configuration.