Unfortunately, your timing makes this more complicated than one would hope.
Smartthings is right now in the middle of a huge transition from one architecture (which was groovy-based) to a completely new architecture. And when I say in the middle, that’s exactly what I mean: not all of the features of the old architecture are yet enabled in the new architecture.
And very specifically, one of the things which is not yet enabled is multi gang zigbee or Z wave switches. 
So there are some 3 gang switch models which have been very popular with the old architecture, but at the moment they all require custom code and there’s no way of knowing if or when they will work with the new architecture.
So…
Since I personally wouldn’t be investing in brand new switches right now if I didn’t have high confidence they would continue to work after a few months, I think the following are your best options
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just wait.
About a year from now we will know a great deal more about both matter support and how smartthings is going to handle multi gang switches in the new architecture. Either of those might come sooner, say in about six months, but no promises. But if you wait, you should have a lot more choices and an easier integration. -
go with lightwave RF. This is a well-established home automation brand in the UK, and the manufacturer does provide their own smartthings integration. This is a cloud to cloud integration, so you don’t have to worry about what’s happening with the new architecture, you won’t need edge drivers for it. (That’s the new stuff that doesn’t support multi gang switches at this time.) They do have a triple gang option as well as some other forms. Standard UK style. They aren’t the least expensive option, but the least expensive options don’t work with the new architecture right now. Lightwave RF does not require a neutral wires for all installations, although then load and features may vary a bit. They recommend a “virtual two way” where one master switch tells another master switch what to do via the home automation network. This normally works quite well with this brand. They also have very good customer support.
- you could try going with Aeotec in wall modules behind the switches, and they do have some models which don’t require neutrals, but I’m not sure you can get three in one switchbox. You might be able to do it with two modules, one which could handle two switches and then a third single, but that does start to get complicated. Well it’s true that This runs into the same issue of the new architecture not supporting multi gang switches, since Aeotec is now the hardware partner for smartthings in the UK I do feel that it is likely that their modules will be among the first to be available on the new architecture. But no guarantees, so there’s a definite risk in going this route. I just think it’s a bit less risky than some of the other brands.
You may get lots of suggestions about various inexpensive models that currently work with custom groovy code. The problem with all of these is not knowing when/if they will work with the new architecture. So that’s an entirely different kind of risk.
I know none of that is what you wanted to hear, it honestly is just a timing issue. A year ago or a year from now there would probably be several good options. I just don’t know what the options will be for the new architecture.
FWIW