Just to bring everything together.
The only smart 3-gang smart light switches/dimmers I have seen in the UK are the LightwaveRF and TheDen. Both require their own hubs and currently TheDen is not integrate-able with any other systems including Smartthings. TheDen do either a 3-gang switch or a 1 or 2-gang dimmer. LightWaveRF do a 3-gang dimmer.
The next common option is to use micro modules like Fibaro, Aeotec, Insteon. Fibaro and Aeotec are Z-Wave and could be used directly with Smartthings but Insteon requires an Insteon hub. These are all available in either switch or dimmer versions and one of the big advantages of these is that you would mix dimmers and switches in the same 3 or 4-gang faceplate. These all would also require using momentary switches like the ones mentioned by @RobinWinbourne
The final option is Aurora AOne as mentioned by @mark_cockcroft this fits on the back of many standard dimmer faceplates but possibly not all of them. This can either be connected directly to Smartthings or has its own hub.
Note: AOne does not have a switch i.e. on/off option only a dimmer option so might not be suitable for all lights.
I am currently leaning towards using Insteon micro modules myself. Insteon have recently announced they intend to launch a new hub with multiplatform support including Alexa and HomeKit. It is also possible to use momentary pullcord switches for use in bathrooms with micro modules including Insteon. Another bonus of Insteon is that they also do plugin switches and dimmers for table lamps.