How can I connect 2 workstations to a multiple monitor setup ?

You'd need KVM switches for that (they also switch the keyboard, mouse & audio of each PC).

But with 3 Displays it can be hard to get such switches, particularly they'll be expensive:

https://www.kvm-switches-online.com/triple-head-kvm-switch.html

How are the three connected at present?  DVI/DisplayPort/HDMI - combination?  If DisplayPort are they daisy chained off each other?  Are you using extended or separate outputs?

The basic solution as rindi says is KVM, multiple monitor support is widely available but you'll need to match the current output or rationalise what you're currently doing to achieve this most cost-effectively.

Example of multi-monitor support via KVM
https://www.kvm-switches-online.com/multi-monitor-kvm.html

Wow that is way over my budget. Those prices one can buy a new computer :)
Much simpler and free is to connect one of the work stations to a monitor and
the other two monitors to the other work station. 

You could of course also get 3 simple KVM Switches that only have 1 Display Output. Those are much cheaper (below $ 50.--). One with VGA connections, another with DVI, & the other with HDMI or DisplayPort (provided your PC's have all those outputs and can use all of them simultaneously). You then just have to change the switch on all 3 KVM's when you want to use the other PC.

For Example, I use 2 Displays. One of them has Display-Port, VGA & DVI inputs, the other has HDMI, VGA, & DVI. I use one cheap 4-PC KVM switch with VGA, & that is connected to 3 PC's & a Laptop.
Then I have a 2-PC DVI KVM switch, & that is connected to the Other Display. The Laptop is connected via Display-Port Cable to that Monitor which otherwise uses the DVI Switch. The 4th PC only needs one Display so that one is only connected to VGA Switch.

So when I change between the 1st two PC, I need to press the button on both Switches. If I change the Laptop, then I need to use VGA Switch button, & the buttons on the Monitor connected via Display-Port to change it input. It is bit more work, but it does work...

Thanks Rindi, To be honest I only need one workstation. My reason for having two is recently I had two failures 3 months apart on my bespoke one with i9, 64GB Ram. Totally it has cost me 3 months lost productivity. I can't have this happen again. Apart from the loss due to time away being fixed it also takes a very long time to install all the software and programs that I use and importing of a load of data from sources and saved in SQL.
This new one will be my main one and the bespoke will be a backup.
I would like to make a mirror image of the main and copy to the bespoke one.
Linking the two workstations is also on my agenda to update files daily.
I'm not all that good at this stuff but I'll get there eventually especially with help from you guys on EE.
Thanks again. 

Mirrors can cause issues if run from other hardware. Product keys won't work unless both Hardware are identical. Different hardware causes problems with drivers & my cause BSODs.

If you need redundancy it could make more sense to virtualize your System (Linux-based KVM, free & OpenSource, VMware ESXi, Hyper-V etc.). That would make the underlying Hardware more or less irrelevant, & moving a VM to other Hardware should be fast & relatively worry-free...

I certainly don't want anymore BSOD's as the last two breakdowns resulted in that.
I have product keys for both units for Windows and Office so I want to keep it that way.

As I'm not a techy type and no contact with one. I may have to go the easy route
and treat each unit as separate entities and just share files via network to keep the data updated on a daily basis.

I can't recall the exact set up we had at past workplaces, but quite a few had this KVM switch, we used a lot of IOGEAR products for our Design Engineers and front desk that had more than one workstation, I don't think we had 3 monitors though for these:

https://www.amazon.com/IOGEAR-2-Port-Compact-Built-GCS42UW6/dp/B000IITQCC/ref=sr_1_6?dchild=1&keywords=kvm+iogear&qid=1628353051&sr=8-6


https://www.amazon.com/IOGEAR-GCS632U-Port-Compact-Switch/dp/B0001BVXI6/ref=psdc_13983771_t2_B000IITQCC

Thanks JMK, I will give some thought to using a switch system to use one mouse & keyboard.
As far as monitors go I would like to just keep one monitor for the backup workstation.
Not sure how I can incorporate that into the setup.

Those are just for a single display. Those are cheap. But as soon as you want more than just 1 Display, it gets expensive.

Can I at least just use one keyboard and one mouse for both units irrespective of how many monitors I use.
I can't see there'd be a problem. ?  I have done that before when living in Japan but just a single monitor. 

Sure, that's the basic function of KVM switches. You just have to make sure you get a model that supports your PC's Mouse/Keyboard input (Older systems use PS2 Ports, while newer systems no longer have those ports, they rather use USB ports to attach the Keyboard & Mouse). It also makes sense to look at what Video Port you need (VGA/DVI/HDMI/DP).

One Work Station has only DP video ports
The other one has mostly HDMI and not sure what other ports if any, till the unit is returned from the workshop.

That's the cabling all sorted. I have just ordered the below. That completes the two monitor W.S.
I already have a HDMI cable for the backup W.S. Thanks guys well done.


https://www.amazon.co.uk/dp/B010SDAXUK/ref=sspa_dk_detail_2?psc=1&pd_rd_i=B010SDAXUK&pd_rd_w=wPfYi&pf_rd_p=62ac3676-b530-493d-9fe9-14410b1870a2&pd_rd_wg=4hsAZ&pf_rd_r=J9J23MC84NN5G94C4DYK&pd_rd_r=ab2b570b-82da-41c6-84dd-811643cbcde0&spLa=ZW5jcnlwdGVkUXVhbGlmaWVyPUExTlgxWVI1TzVaTFNUJmVuY3J5cHRlZElkPUEwNTcxNTc0MTVDSUIyM1dFUjk4RiZlbmNyeXB0ZWRBZElkPUEwNTg4OTk3MktJSUNFRzlGVEZaQSZ3aWRnZXROYW1lPXNwX2RldGFpbCZhY3Rpb249Y2xpY2tSZWRpcmVjdCZkb05vdExvZ0NsaWNrPXRydWU=

Thanks guys for your help as much appreciated.
I'll forget the monitor idea as too complicated and costly.
However will use the keyboard mouse switch as recommended by Jazz
and which I have used on a couple of occasions while residing in Japan.
I've bought the mini dp to hdmi adaptor recommended by Rondi. So
I'm all set to go, just waiting for my second workstation to return from the menders.
Thanks a lot you good people.
Ian

Nice! Glad you got it sorted out. You're welcome! :)