That's really sad that you lost the one Cory like that.
Don't stress about your lone Cory as he'll be fine until you get him some buddies.There have been many people over the years that have bought just one Cory because they didn't know any better,and the fish did okay.
Buying at least 3 of them just makes them feel happier and more secure within their group.
I have owned tons of Cories all of my fish life,and they really do love being with their "buds"
so the lone little guy will have a big grin when you do add the other 2.
Good luck with them.