I have done T5 to T3 before. Just follow the purple sings and you'll be put on a bus. Make sure you have a copy of your itinerary either on paper or on a device as there's a check before you're allowed to proceed towards the bus. The queue for security to get into T3 was a little longer than I'd have liked but nothing outrageous.

I am doing the same thing in Feb with the family in tow and it's not giving me any concern.