I have recently flown with Virgin from London to Hong Kong which means I had the pleasure (?) of departing from T3. Unlike T5 which still gleams of newness, T3 is very rough around the edges and not that pleasant a travel experience.
Firstly it's a myriad of elevators and walkways from the carpark to actually enter T3. Virgin has a huge area for check-in which can be a bit of a bun fight as there's no easy queueing system. Unless you are flying Upper Class, where you are whisked away to a secret swanky lounge and your own security, you then trudge around the corner to join the masses at security.
On seeing a lone, anguished parent and a screaming child, a friendly face appeared and ushered me through to the 'Family Lane'. Big mistake.. huge... If you have this choice, don't take it! I promise you, even if the queues for security on the other lanes seem longer, you will get through faster. In the Family Lane, everyone is taking 10 minutes to unbolt their stroller, tend to screaming children, taste all their milk etc
Once through, there are the usual shops and a few good eateries. There is no designated children's play area that some of the other airports have. As mentioned in another post, I recently received an 'Airport Angel' card through Natwest and I had googled to see what lounge that would give me access to.
The No 1 Traveller lounge is pretty good, nowhere near as posh as the actual Business Class lounge but there was free food & drink and lots of room for my baby to crawl around. If you are travelling with older kids there is a family room with a TV and table football.
Here is Heathrow Airport's Guide to their facilities across all Terminals: