My guess is that Wladzia is short from Wlodzislawa (or from any other name that starts with Wlod - to rule). Similarly, Bronia can be short from
Bronislawa or any other name that starts with
Bron - protection. I honestly have no idea about Marysia, though you could be right about
Maria. For more ideas check the polish names page
http://www.behindthename.com/nmc/pol.html