All three saws will have similar perfomance, weight, and durability. The Husqvarna will be the most modern design. The Dolmar is the lightest and has the largest displacement. I have not had an Echo CS-520 in my hands, but did own a CS-670. Well built, and I have no problem recommending one, although it may not be up to the power to weight ratio of the others, and Echo doesn't advertise their horsepowers.
To me it would come down to your individual preferences and dealer support. If you like to follow the masses, get the Husqvarana. If you like to be a little different, buy the Dolmar. If you like having widespread parts and service support, get the Echo. I don't think you'd be unhappy with any of them.