You should be fine
As long as you get the receipt before the due date of your first 3-year H1, you should be fine.
my case.
First H1 expired date: 10/30/2003
I filed H1 extension, and receipt date is 10/20/2003. I got H1 renewed on March/2004.
But if your H1 extension is rejected, it will be another story.