It's likely that while his software (mind/memories/personality) might be an adult, it's now running on a 3 year old's hardware (brain/hormones/chemicals), so he's no longer as capable/mature as in his prior life. Or so I hope the story goes. The age regression/reincarnation stories where the young brain affects the MC's new mentality (and the MC having to come to terms with that) to be better/more compelling than the stories where the MC is basically just a tiny adult people mistake for a child.