From what I have heard you would still do the three years unless you do some hybrid program like the above posted said. Mainly because every school does their curriculum in a slightly different order - there is no set schedule for all the 1st years in every program so it would be hard to fit a PTA in to certain PT programs as they could miss very big things. I would contact school that you are interested in and see what they say.
Also if its DPT you want apply for that because doing PTA you're gonna do 2 years of school then what in a few years apply for PT then its another 3 years, why not take a year or two now to get everything to apply right in to a DPT program if in the end you want to be a PT?