Keep in mind that the FP only gets you to the merge point near the bottom of the stairs. There you join the people who came from the standby line and head to the final boarding area to be placed on a ride vehicle. It will usually take 5-10 minutes to get boarded even after you get past the merge point.
The time it takes to get to the merge point is going to vary based on how many people arrive with FPs in the 10-15 minutes before you. We have never taken more than 5-10 minutes to get to the merge point out of about a half dozen uses of FPs since FP+; sometimes we walk right up to it. But, others have reported encountering long lines in the FP+ queue. I have heard some people say that they waited 25 minutes with a FP, but they usually don't say if that is 25 minutes to get to the merge point or 25 minutes to get boarded.
if you arrive at 3 PM you should be fine to get out and get to your ADR at 3:45. But, if it was me, I would be getting there a little earlier just to be on the safe side.