How to open emulators from android studio 3.0.1
Open Emulator
Android Studio.
Check the picture. Android Studio 3.0.1
Check Image
Click on AVD Manager, new Screen Will appear. So choose emulator if you already have any emulator or Create Virtual device and clone it.
It looks like you haven't installed Android Emulator option in SDK tool,
Go to File --> Settings, Now follow the image and install Android Emulator.
After installing, restart the Studio and your problem will be solved.