How do I write any math symbol in Sketch 3? I want to write them in "text" if I can. If I can't, what else can I do besides of insert pictures? Here are the pictures attached.
Try the Character Palette
If you can insert it via the OS X character palette, you're good.
Other than that, I'm not aware of any math plug-ins for Sketch.
Roll your own
One of Sketch's most lovable features is that you can write plug-ins to do most of what you can dream up. Given math symbol support at the OS level, it seems feasible that a UI could be built to handle the input.