need help wont shift into third

Sounds to me like the people that installed the trans. are the rip offs. I bet MadDart's right, they cleaned things out and that's why it shifts now and they told you it wasn't full of crap because they knew they screwed up by not flushing things out. It probably did need a converter due to a worn or cracked neck and due to incompetence they overlooked it and since you know practically nothing about how a transmission works you'd believe their crap/lies. Don't mean to be rude, just telling it the way I read it