Terminal 3 is large, consisting of several piers. You will have to go through transit security and may have to take a train but probably not a bus. Perhaps the moderators could move this to the Emirates forum, where experts could advise on details like minimum and advisable connection times, likely wait at transit security around your flight times (it's usually pretty short), the likelihood that you will have to stand in another line to get your boarding pass (low; they'll likely give you the onward boarding pass in London), and the availability of mobility assistance.