From what I can see I would say that all three were males, C. sterbai are not the easiest to sex as both sexes have very similar body shapes. Looking at what I can see of the ventral fins they look a little pointed which would favour them being males.
The ventral fins on the right cory is fairly round. I couldn't get a good picture of it because it was always moving in the container. The other 2 steabai sat motionless which helped a little.
Here's a trick I use to get a clear look at ventral fine when I'm having difficulty in sexing a species.
I use a small show tank with an extra piece of glass inside angled up from the front to the back, then when the Cory is placed into the tank it cannot sit directly on the bottom and the ventral fins are in full view.
I added a close up photo of the fish. This is the best I can do at the moment without getting a display tank + a piece of glass/acrylic at a store. HTH