DryDis powder, great except it stains your hands red. Does mean you know which pens you've done before you put the fresh straw down, though. But, if you only have three and there is room/enough gates etc. I would just have three separate pens ready, then you can just disinfect them when the ewes go out with whatever you use in the house. Post lambing, ours are all washed down with something like Milton, just whatever I have in the house at the time.