As i understand one dot is soloable bosses like quest bosses,
two dots are group bosses typical world bosses, you can solo some of them depending on mechanisms.
3 dots are dungeon bosses, not soloable unless you out level the content a lot.
Why does the solo dungeons in Cyrodil have 3 dot bosses, and how could I solo it, yes I'm VR13 still he was not much harder than an VR10 one dot boss.