It depends how big your 3-year-old is, but mine were way too big for the virgin child seats by that age (though mine are big and tall for their ages). They used them from 4/5 months to about 18 months, then just sat in the normal seat with pillows etc.
Unless the design of the seat has changed much, it doesn't really recline, so is not comfortable for sleeping in. It was useful for keeping the younger one strapped in while I took the older one to the loo etc - I suppose if you have a smallish, wriggly 3-year-old, it could be useful to keep them in one place while you change the baby's nappies and so on.
There is probably no harm in booking one, and if you try it out and find that it is too small or uncomfortable, the crew will just take it and stow it away again.
Other tips for getting 3-year-old to sleep - take pyjamas for them to change into, take your own little blanket or mini duvet (my two had cot-size duvets they took on flights with them until they were five or six), and a cuddly toy or two (soothing, and also extra padding as the inflight pillows are small). Restful music on an ipod (with toddler-friendly earphones) can also be useful.