If you are taking all the children together that is quite hard to combine 12 and 3.
I recommend using games, such as the ones shown on the demonstration video on the link below.
The first game you can see is outside with a scarf and one child, but this game works well with three children too. The only thing is it would be rather too boisterous for the three year old.http://www.homeenglishteacher.com
For the three year old I recommend these games and stories with songs, which the six year old will also enjoy:http://www.amazon.co.uk/ESL-Stories-Pre ... 1482012081
They are also on Amazon.com, Amazon.es, fr, it, de and co.uk.
For the six and twelve year old I recommend plays and skits in action, these can be done outside but would be too much for the three year old. You'll find them on Amazon by the same author of the stories above.