Ok,
You are exhibiting much anxiety over this. Let's just think clearly for a minute about your situation. My opinion will come later. You think that your plant is rootbound. In other words, you think your "roots stifled or inhibited from normal growth, by the confines of a container."
www.stealthgrow.com/glossary.htm
If you really think your plant is rootbound, then putting a rootboud container into another container is not going to help. A few roots would poke out of the bottom of the original container and try to poke around the new one as much as they could. This would be a waste of your time and effort.
I personally don't think you are rootbound.
I bet that you prepare your 5 gallon pot, flip over this 3 gallon one to get the container off, and the medium crumbles as you try to stick it all in the new pot. That is, unless you are really rootbound. In that case, the root mass will hold your medium together very well. I doubt that a plant with a main stalk smaller than a pencil will have have used a 3 gallon container enough to even hold a root ball together after three weeks.
Anywho, let us know what happens and take pics.