Aside from the fact I don't think a sequel to FF13 is a good idea having them work on FF13-3 before FF13-2 is released doesn't seem so imprudent to me. You don't have 100% of your production teams working on a project at any given times. The people involved in the early aspects of FF13-2 are probably ready to move on to a new project, hence FF13-3.

What they should do, and always do, is take a leaf out of FFIV's book.
And here I was thinking you meant just move on to FF5.