While it's recommended, a witness is not required to fill out a CF98, and whoever told you it's no longer valid after three months is wrong.
If an injury occured, and your chain of command hasn't ensured you've filled one out, they've erred, but you're still able to fill one out.
Attach any pertinent medical documentation in lieu of a witness statement, and you'll be fine.
You know what I would really love to see? A class added to PLQ on how to fill out a CF-98.
Better yet, add the safety management course, which as far as I'm concerned is an absolute must for any supervisor or leader, as a module to the course. If they're not going to do that, at very least, a class on PLQ on how to fill out a CF-98.
How many forms have been filled out wrong, or not filled out at all, because an individuals chain of command didn't understand the CF-98, and/or how to properly fill it out and send it on it's merry way.