If it is a single APK file named "Windows 3.1" that promises to "install instantly" without requiring you to provide your own disk images (from DOS and Windows), it is fake. Real emulation requires configuration and original files. Fake apps require none of that.
Place the Windows 3.1 setup files into a subfolder (e.g., DOS/WIN31 ). : Open your DOSBox app and mount the DOS folder. windows 31 apk