OK, I don't know about flashing stock ROMs on Samsungs. I think you use a program called Kies, and can obtain the software from Sammobile.com, but that's as far as my knowledge goes. Make sure you find the software for your exact model, and if you have any uncertainty wait until someone who knows these devices replies.
But you definitely don't flash stock software the way you'd flash a custom ROM. That's true on any device. So to remove any ambiguity, can you tell us what tar it is you found, what you mean by "a regular ROM" (custom or stock?) and how you would flash it? It's easier for people to help if they don't have to make guesses as to what you mean.
But if you are rooted and have a custom recovery - and I can't think how else you could erase your ROM by accident - why not just flash a new custom ROM? Or find a stock ROM that can be flashed from recovery (most likely you'll find one in the same XDA forum that you could find a custom ROM)? Again, and as always, make sure it's for your particular model, since there are different models of S3.
