Hello Eileen,
Olly is on holiday at the moment, so I hope you don't mind me stepping in to answer your question.
You will have to get yourself from Terminal 5 to 3, but it is very easy and quick and wont cost you anything. Below is the instructions from the Heathrow about the various ways you can get there... (Heathrow Central is where Heathrow Terminal 3 is)
Use Heathrow Express trains for a free transfer between Heathrow Central and Terminal 5. This quick, convenient service runs every 15 minutes and the journey takes about three minutes. Please ensure you board the correct train as Heathrow Express also runs non-stop to London Paddington.
Heathrow Central is between Terminals 1 and 3, which are a few minutes' walk away via underground walkways and are signposted from the platforms. The Terminal 5 rail station is in the basement of the terminal and linked to check-in and arrivals by express lifts.
Alternatively, the 441 bus provides a free transfer between the terminals, taking 10 and 20 minutes. The N9 night bus, which runs between 23:55 and 04:55 (times vary at weekends), also takes ten minutes.
Services stop at the Terminal 5 bus station (next to the terminal) and the Heathrow central bus station (between Terminals 1 and 3).
http://www.heathrowairport.com/heathrow-airport-guide/travel-between-terminals
Sally-Anne, your editor