I have a galaxy grand prime.when i try to reboot into recovery mode with 3 button combo, clock mode dosn't start. it booted normally.how to solve this.
On some samsung devices (rather newer, e.g. Galaxy A3), there is no need for root access to reboot into recovery mode.
Enable the Developer options on your device (Settings > About device > pushing several times "Build number") and switch on the USB debugging.
You need to install adb http://lifehacker.com/the-easiest-way-to-install-androids-adb-and-fastboot-to-1586992378/ and to run the following command:
adb shell reboot recovery
This should reboot your device into recovery mode without root.