Short answer: Echo -> TSR -> Arches
BUT
To my mind it is good idea to start with Arches, regardless of Arches being the shortest game so far. (I want everyone to know that I'm not complaining, this VN is in my opinion tough 10/10 with plot, ost and characters) The problem that I had reading Arches was that I knew the main antagonist, some ghosts/spirits and every location, it might sound funny (for me it was xd) but when I was reading it I was like: oh, Cam and Dev are going to the ****, I wonder when they will ******. -5min later- There you go, knew it :)
For many people it might be good actually: to be so familiar with Echo town that you know in advance a lot of things that might happen to the cast even before they will know it, but I don't like spoilers and Echo VN is spoiling a lot. (I am aware that this familiarity could be deliberately implemented by creators, however not everyone has the same tastes.)
Second is Echo, no comment here <3
Third is TSR bc there is a few characters and events that won't ring you any bells. TSR is great in regard to expanding Echo world, but not necessarily learning about it from there.
Again, this is MY opinion why I would go for Arches at first, but if those aspects don't bother you then stick to my short answer up there :)
No matter the order, I'm telling you you won't regret any second spent in this world and that is something that everyone will tell you.