>>1906717It's not about the cage being long
practically any cage will be long enough
If the cage is too short, the granny chainring becomes only useable with the lowest gears on the cassette. That is only when you would use it anyway. Failure, rubbing, is non-catastrophic.
In this sense, the limits of the derailer are a non-issue. It's similiar for the limits of a rear derailer. So you can't crosschain small+small. So what?
The correct derailer is one where the curvature of the cage closely matches the large ring, so that it can be set closely and you will achieve the broadest crosschaining before rub, and the crispest shifting.
Also that the middle groove in the triple specific derailer matches to where the middle ring is in this position.
It's really nothing to do with the granny. Those are fairly generic ratios and many/most shimano mtb derailers will be appropriate. Setting up front shifting however can be quite tricky and can require experimentation. If you want to make it easier for yourself you can run it with a friction shifter.
It's also about fitting your seat tube. Yes you can shim but it's better not to. And, if indexed shifters, suiting that indexing.
