The safest method would be to have Fl 3 and Picc just be separate players (but sharing a part layout), put in the instrument changes manually using text and hiding all the unused staves. You’ll still have to watch out a bit with your system breaks in case the change happens rather quickly—ideally the two instruments should never share a system. I do something similar in case of A/Bb clarinets.
All of this is obviously not ideal in case of rewrites/reshufflings because you’d likely need to redo the instrument changes and re-babysit your system breaks.