I have downloaded debian-9.3.0-amd64-DVD-3.iso from internet. I just opened it with help of File Explorer and copied its content to a usb. I tried to boot my pc with that usb, but my usb was not detected in the boot menu and I did the same while installing ubunbtu, ubuntu media was detected.(i installed it and used it for 3 months). Where did I do wrong with the Debian?
You can not simply copy the contents of the ISO that File Explorer shows you to the USB stick. The ISO also contains a boot sector (sector 0 of the ISO) that allows the computer to boot from that ISO image, which contains the address of where the bootloader is to be found on the ISO etc.
You need to follow the howto proposed by @roaima or similar to create the bootable USB drive.
Answer from the Frequently Asked Questions (Debian FAQ):
Additionally to the method above for Linux systems, there is also the
win32diskimagerprogram available, which allows writing such bootable USB flash drives under Windows. Hint:
win32diskimagerwill apparently only list input files named *.img by default, while the Debian images are named *.iso. Change the filter to . if you use this tool.
Please note, that Debian advises not using
unetbootinfor this task. It can cause difficult-to-diagnose problems with booting and installing, so is not recommended.