Welcome, welcome all new and delurking forumites.
VernonHedrick wrote:I sold a novelette a couple of months back. Am I correct to think that I could still sell THREE short stories before I am no longer eligible?
A couple of thoughts---
First, the guidelines suggest publish, not sell. A fair few prize winners got their win after selling their last eligible story, but before its actual appearance (with final appearance happening after quarterly submission, but before actually winning four months later).
Second, that is professional publication, which is to say minimum pay of six cents a word AND distribution of five thousand copies (or views for electronic publication).
Third, the guidelines
do say "...more than one novelette, or more than three short stories..." which argues (as you posit) that so long as you don't exceed either requirement you would remain eligible.
Finally, the contest exists to discover pro-level talent, with winners increasingly being people who are starting to sell regularly to pro markets. The contest administrator wants to be convinced, so if you're an edge case but make a good argument (and write a prizewinning story) arguable requirements can often be successfully argued.
Go for it. The best answer is just to win.