My FP 3+ charging unit needs replacement. Does the unit in my old FP 2 fit in the FP 3+?
No, that will not work at all. Doesn’t fit both physically (shape) and technically.
However, you might fetch a reasonable price for the FP2 bottom module (if its microphone and USB port still fully work) if you offer it in the market category of this forum.
What your FP3 needs is this: