Once you have your fourth and are working, I believe you can write the part 'A' of your third right away....when you have enough steam time, you can write the part 'B'...... There is no correspondence involved.
If your workplace is fairly big, there will be a library of old test questions floating around that you can practice on. Also, buy the 3rd class module package from SAIT, and get to know this website
http://www.sopeec.org/
Good to see some fellow power engineers on this board!