After letting everything cool, I transferred the "3" to my cake board. I slid a cardboard cake board under the silicone baking mat and then put another cake board on top, and carefully flipped it over. I peeled off the silicone baking mat from the bottom of the cookie and then put the cake board back on top (which was really the bottom) and then flipped it over again.
-- As a side note, I used scrapbooking paper from my stash to decorate the cake board and then wrapped it in Glad Press and Seal. The paper and the Glad are taped on the back with packing tape. Using the Glad is a must because if you drop icing on the paper, it's not coming off. --
I ended up using an awl that I had bought, thanks to a recommendation I saw on a cookie site. It worked perfectly. I carefully scratched a dashed line in the base coat of icing.