I think you'll be fine. Try not to chill them too much. In the magazines they sometimes suggest taking the media out of cannisters and putting it in a bucket of tank water. For a three hour move I wouldn't be too worried about it. I'm sure I've turned my filters off for photos and forgotten to put them back on again for three hours, and not noticed any effects!
Thanks for all your replies, they have been most helpful! I think I can stop worrying too much and will be in touch LD if and when I move so I can borrow that battery operated air pump. Luckily logistically it is not going to be too difficult since my bigger tank is only 125 litres and easily moveable. Exciting and scary prospect this moving at the same time... :?