If you plan on having tickets left over for another trip in a couple of years a ticket for your 3 yo that you just use as you need may be advantageous. I had leftover children's tix from 2001that had originally been used by my now 14 and 11 yos. It would be ridiculous to try to pass them off as children just to use the ticket so I asked at Guest Services. They upgraded the tickets for no additional charge (with all the days and pluses intact) to adult tickets for them. This was great and saved us some real money. The idea is that if the tix are truly non-transferable kids do get older and eventually the original owner of a child's ticket will grow to adult age eventually.
One other point, the finger scans appear to be phooey!!! We shuffled our passes around (they were all bought at different times on different trips originally) amongst our family for the whole week with not a scanning issue. Maybe it was because they were old passes and maybe it's only a random check, sometimes they do or not. Or just maybe it is to scare you enough to not ever think of using someone elses pass. I had no guilt, I bought all my passes for use by my immediate family and that's all that have ever used them.