I'm currently on 3.15 but I haven't got the play TV system. I have managed to update game titles without the latest firmware updates being forced on me. It does seem to vary though, some PSN titles download an update then prompt you to accept the latest firmware update.
If >3.15 is needed by the software you are updating you are always asked to confirm you definately want it. You could always try to update & see how far you get. I have never had firmware forced onto my system when updating software.