Mh, I can see Mary being pronounced in multiple ways, but the other two sound the same to me and one pronounciation option I have in my head for Mary sounds like them, too.
Fascinating, that stuff, though.
I actually have sound references for how I think swan and lark sound - the online dictionary I use has these little play buttons to play someone saying the word, and that's pretty close to how I think they sound:
swan
lark
These seem very different to me, the lark is similar to a german a, while the swan has a tilt towards german o in my ears.