dell inspiron 9300 does't turn on

hi, trying to work on my old inspiron 9300 while unfortunately also waiting to fix the alienware m17x. Suddenly the dell inspiron is not turning on anymore, the power LED lights for 3 seconds then turn off again, doesnt continue to boot startup . I changed adapters, tried with and without battery, tried the static power even for 5 minutes. I opened the laptop, removed 1 memory slot at a time then power on but didnt work. removed the CPU and GPU, bios battery, and made sure they are correctly all re-slotted perfectly, but even then the laptop refused to turn on. Heard some changed the motherboard without any success. Any more steps to follow as i badly need it for an urgent job. thank you