All in One Options
In addition to lightwave RF, there are some other all in one multigang options, where you just swap out the 3 gang smart switch for the dumb switch, and in theory, this should be easy as long as you have neutral lines at the switchbox. (I believe all the brands except lightwave RF require a neutral.) There are available three gang Z wave and zigbee switches.
Unfortunately, the smartthings platform has never implemented what are called “multi channel“ or “multi endpoint“ devices very well. That’s where there’s a single physical device but it has three or four or whatever individually controllable options. So the ones that you would expect to work, often don’t. And there isn’t really anything we can do about it. 
There is supposed to be an upgrade coming on the platform side for Z wave multi channel devices, but it’s not here yet.
WiFi Switches
So… That leaves Wi-Fi devices. if you go to Amazon.co.uk you can find lots and lots of three gang Wi-Fi smart switches. They vary in quality and in safety certifications, but Sonoff and Meross are two good brands.
All Wi-Fi switches will have their own app, typically Chinese, and they work fine with that app. The question is how do you get them to work with smartthings? They don’t typically have an integration and there’s no industry standard for home automation devices for Wi-Fi the way there is for Z wave and Zigbee so many of these only work with their own app.
So now you have to start looking for a “man in the middle” to deliver integration. That could be IFTTT, it could be Amazon routines, there are a couple of ways of doing it.
SONOFF
People with strong technical skills used to be able to “flash” the firmware on sonoff devices To allow them to be used with smartthings without Requiring going through the Chinese servers, but I’ve heard that that no longer works with the newer models. If you’re interested in that, you can check the forums for the most up-to-date information
IFTTT
IFTTT can work well as long as The Internet is available, IFTTT is working, and the smartthings cloud is working. But that’s a lot of points of vulnerability for a light switch. You may be fine with that if it’s a situation where you’d be happy to go over and press the switch on the wall if the automations weren’t working. But it’s something to be aware of.
Also, some models offer two way communication and some don’t.
With one way communication, you could use smart pthings as the “IF” at IFTTT and the Wi-Fi switch as the “that“ and you would be able to turn the switch on and off from SmartThings. But if someone turned on the switch at the wall, smartthings would not know that the state had changed.
With two way communication, the Wi-Fi switch can be either the “if“ or that“ so you can create rules where if the Wi-Fi switch is turned on at the wall then smartthings gets notified of the change in state.
It’s up to each individual device manufacturer whether any given model has one way or two way communication, so you just have to check each one that you are looking at.
Amazon Routines
Amazon routines can give you some integration with devices that would otherwise be unavailable to smartthings, but it’s almost always one way. So it may be useful, it may not. And again, you’ve added a lot of points of vulnerability for a light switch.
Summary
As of this writing, LightwaveRF Remains the easiest way to do this. Their switches don’t require a neutral, they’re reliable, they’re safety certified, they have an official two way integration. It’s just that they are expensive compared to cheap Chinese devices.
There should be quite a few zwave or zigbee alternatives, such as the ones from MCO home or Schneider, but unfortunately, the smartthings platform just doesn’t handle this configuration very well at this time. They may do better in the future, or there may be some individual models which work OK. Hopefully if anyone has one that does work, they’ll add to this thread.
Aurora One has an official U.K. integration for a two gang zigbee dimmer, but don’t offer a three gang.
So that leaves Wi-Fi switches. There are lots of these, but most are not very well engineered and don’t have safety certifications. And they don’t have official integrations with smartthings. But there are two brands, Sonoff and Meross, Which are better engineered with more safety certifications and which have been fairly popular in the community. The Sonoff offers the most integration options, but only for some models and you have to have some good technical skills to implement it.
Otherwise, regardless of the Wi-Fi brand, you have to start looking for a “man in the middle” option. IFTTT is usually the easiest of those, but may only offer one way integration.