You can download the opacity maps below for the following clothes included with CC3 to change the length of them.
Dress A ( or any of the Dresses included with CC3), Tunic Long Sleeve, Rocker Jeans, and CC2 Shirt or iClone 6 or 7.
If you have purchased the slacks from Reallusion, you can download an opacity map I created  to change the length
of the slacks to short pants.
Look at photos of before opacity map and after opacity map.
It gives extra wardrobe for your characters, without having to create or buy them.
Make sure to check the Alpha Threshold box (about 50) in modify panel when highlighting clothes.
On the CC2 shirt, uncheck the hide inner mesh box when highlighting the shirt.