Junction Box

Due to remodeling, a 3-gang switch box will become obsolete. I will still need to make use of the wires inside; they will just be re-directed to the site of the new replacement switch box.

Question: can I use the existing 3-gang box as a junction box, or does a junction box have to be of a certain type?

Also, assuming I can use the existing box, how do I cover it? At the store, the only covers I saw were the 4" square and 4" round covers used for traditional junction boxes. Or do I just use a blank plastic plate that is viewable from the room? (btw: do they come in the required size, i.e., about 6-7" wide?). I'm not worried about appearnace as it will face the workshop in the garage.

 

I think at larger stores and supply houses they have different cover plates.