If your 3 year old is anything like mine -- stickers and/or sticker books -- these are a must !!! Sainsbury's do brilliant sticker book collections (various themes but each has different activities in as well) where you put the stickers onto the corresponding pages. Keeps her quiet for ages

We have a PSP which you can put different short kids programmes on - Charlie & Lola is her current favourite followed closely by Lazytown. They are always good for the aeroplane. We have also invested in a Nintendo DS (for the whole family but do I get a look in ?) and have bought Nintendogs for her. I think she is still a bit young for it but it does seem to make her sit down quiet for a little while to "look after" and "walk" her dogs
Last year, she wasn't that bad during the journey, although it was only to Menorca. We had stickers on the way out and the PSP on the way home. Heaven.

There is another thread HERE with lots of tips for stressed parents