I wouldn't worry about this at this time. I agree with the person who posted that the other boy to whom you refer sounds advanced for his age, rather than your stepson sounding behind for his age.
Your stepson might just have no concept yet of what "today" really means and answers with his random list of stuff he has done (or wants to do, or heard other people saying they did) with no relationship, in his head, to the idea of "today." That would not necessarily indicate a developmental issue at all, unless you see other signals alongside it, and you do not say here that you see any other signs of developmental delays.
It sounds from the post as if he does speak well enough, in terms of putting together sentences that are appropriately complex at three years old.
Have you tried giving him directions and seeing how he responds? A child, by three, is usually able to follow a basic two-part direction such as "Get your book from my bag and bring it to me" and so on. If he can handle those kinds of directions OK he's on track.
I would be more concerned about a three-year-old who wasn't putting together sentences at all, or who had speech issues, or who couldn't follow a simple direction, than one who just didn't get the idea of "tell me about your day today" yet.