Unfortunately, there's no perfect solution if each site with an NVR is on a separate LAN to the TVs...
Not a great option, but probably the simplest, is to add them all to a Hik-Connect account, and then sideload the app onto a Firestick or Android TV box for each TV:
Has anyone used an android device to playback hikvision cameras on their TV? If so, what device do they recommend?
www.use-ip.co.uk
There are lots of YouTube videos if you search around "Hikvision APK".
But, you might encounter some complications, as the app is built for Android tablets and phones - the feeds will also automatically time out after 5 minutes (no way around this, it's just a limitation of Hik-Connect) - all discussed in this thread:
Has anyone used an android device to playback hikvision cameras on their TV? If so, what device do they recommend?
www.use-ip.co.uk
I'd test it on a single device first before buying one for each TV.
I'm also not certain whether you will be able to log into the same Hik-Connect account on three different devices to stream each NVR's feed on different TVs.
So, you may need to create three accounts and share if from the main account to each of the others.
Or, if your client is happy with using just two TVs and has a PC nearby with multiple HDMI / display outputs which you can connect to the TVs, then running
iVMS-4200 on Windows is an option.
You'd need to add all 3 NVRs to a Hik-Connect account again, and then you can log into that account in iVMS-4200 by clicking on the cloud button in the top-right corner:
-Log-in to the iVMS-4200 and on the top right corner, select the Cloud Icon that says "Not Logged In." (see 1st snip below). -Accept the pop-up window that says you will be entering Hik-connect. -Then log-in with the Hik-connect username and...
supportusa.hikvision.com
You can split the software across 2 screens to view an NVR on each:
Question: Can IVMS-4200 be used on multiple monitors? Answer: Yes, you will need to hit the three dots on the top right and select "auxiliary screen preview". This will create a separate window that you can display on another monitor.
supportusa.hikvision.com
I've never done it myself, so there may well be caveats that I've not thought of!
Someone else might have a better or tried and tested solution