3-5% is standard shrinkage for a cotton batting. Is this an all-cotton Hobbs batting? If it's 80/20, then I honestly would not worry; I've never gotten more than a soft 3% or so shrinkage. I have heard of other people presoaking (not washing) Hobbs 80/20 and drying in a dryer to shrink before layering, but I've never done it.
If you don't want any shrinkage, I would switch to a polyester batting.