1) Nice clean shot, though I find the composition a bit bland what with the subject being dead center. I'm guessing it was just natural lighting? Either way, it's good. The column looks slightly tilted, which is a pet peeve of mine, thought it could also just be the lines playing tricks on me since the columns in the background look straight. Rotate 2 degrees counter clockwise and see how it looks. Move the cosplayer's bag out of the frame.
2) The shadows on the face are a bit harsh, but it works. Was this flash or natural light? You could consider adding a flash on camera-left to give some rim lighting and highlight the edges of the hat, similar to what's going on with the brim of the hat on the right side. Crop up just a tad to get rid of the random thumb sticking up at the bottom.
3) Good lighting on this one. You can see what a hint of rim light on the subject does by highlighting the right edges of the hair and body on the right side, which makes her stand out from the background that much more. I don't know if I'd keep the knife hand raised like that since it has no support and looks to be floating. You could either pull out a bit to show the elbow, which connects the knife hand to the rest of the arm and gives it placement, or just drop the hand and not show it. Alternatively, you could raise the arm over the head and still show the knife and rest of the arm.
Thanks for sharing and keep up the good work!