You can quickly reset a Samsung A3 smartphone by booting into recovery mode (e.g. when you're locked out):
Turn off the phone by holding down the power button (right side) and then select to power off.
Hold down Volume up, Home and Power buttons to power the device on. Keep Volume up and Home down until you see the Samsung logo. After a while you will get the recovery menu.
Use the volume buttons to navigate in the menu (up for up, down for down). Select wipe data/factory reset, then press the Power button to confirm. Use the same keys to select yes if an additional prompt appears.
The reset takes a very short time after which the recovery menu will be displayed again. Either select reboot now or power off to reboot or power off the device. The next time the device boots, it will boot up with factory defaults.