Is it just me, or does Duelist/Shadow/Ranger seem like the same class?

No it's not just you.
My first CI phys crit bowman was a Shadow (Domination league).
Then I did CoC CI phys crit bowgirl a Ranger (Anarchy league).
Now one of my chars in Rampage is a life based ST crit dagger guy. A Duelist.

Yes there are differences in those 3 chars, you mentioned, when you play life or CI build, melee or projectile type.
But every single time I played Critical + Projectile + whatever other variation on top of those two I basically ended up with the same passive tree spread from Duelist through Ranger to Shadow and any of those 3 could have been picked for the build.