A box, big enough for some nice polar fleece blanket and 3 cats, not too much bigger than that would work.  Seal up the box and cut an entrance hole and put the blanket in there to make a little "cave'.  They should all crawl in there and snuggle to keep warm. 
I just always worry about heaters...any portable heater.  Fire issues.  
Put the box in the kennel with a litterbox and food/water.  You'll have to find a way to keep the water from freezing though.