Accidentally wiped /system. How do I fix this?

Hi! So friend 1 had asked me to help him root and flash CM on his S3 and I gladly helped. We were having a tough time doing it although I did it quite easily on friend 2's s3. Friend 3 chipped in to help us out by asking us to use Kingroot and let it root the S3. I thought it was ok and we let it do its thing. After it was "rooted" we tried flashing CM. There was an error during the process and it said it couldn't read the binary or something like that. I then wiped /system through TWRP thinking that it would revert everything back to stock. Well, obviously it wiped Android. I am so bad with Samsung phones xD Unfortunately, I did get ahead of myself and did not backup the phone when it still had Touchwiz. Good thing is, I can still access TWRP and download mode. I've already downloaded a stock rom for the s3. Now, how do I flash it? Are there any other solutions?
Thanks :)