This is a really common issue, especially when trying to install multiple Smart Switches into a relatively shallow switch box and worst case when you have a triple gang box to accommodate (with all the wiring for three separate switches plus the added depth of three Smart Switches, even a deep box can be a challenge!)  

Here is a fairly simple way to extend your available depth, without having to replace the box. This gives you an additional 5/8" depth 

(these are not my images, they are pulled from the Amazon Listing) 

3 gang switch box extender

I believe they also make a 2-way, although the 3-way can also be converted by just removing the centre section

Yes, it is not inexpensive but does give a simple bolt-on solution which is obviously much less labour-intensive than removing the original back box and installing an old-work deep box in its place. 5/8" gives a good bit of extra space, while not looking too overly protruding.  

I think it's a quite attractive solution for what it provides and certainly affords the flexibility to utilize multiple Kasa Switches that might otherwise be prohibitive. 

Personally I think this style, where the extension is slightly bigger than the cover plate gives a more pleasing aesthetic than the Levition surface mount boxes which are significantly deeper and also have the cover plate slightly overhang (which does not look good IMO!) 

3 gang Old Work box is the alternative if you have the time and skills to execute - this would be my personal choice and I have indeed utilized this type of replacement for a 2-gang location - it involves removing the original 'new' work box which would have been most likely nailed to the stud - that of course retains a std flush mount switch cover plate.   

If anyone has other solutions, please post your replies with suggestions for the community.