Solved Pc won't boot

Hello everyone
I have an older PC, using 775 socket, ddr2 etc.
It started few days ago
I was able to boot PC, but if I opened anything it immediately shut down, I was monitoring cpu usage in task manager and it was changing every half a second from 90% to 1% usage
When I realised I cant open anything, I just let it idle for few minutes and it would work for 3-4 minutes and then it would shut down.
I've read some articles that I should put ram in another slot and try it again, but it didn't help.
I cleaned the dust using air compressor.
Now it starts booting, ventilators start spinning and after few seconds it shuts down and it automatically repeats the same thing until I plug it out of the wall. Also there is no booting animation on monitor.
I hope I provided enough info and sorry for language mistakes.
Thanks in advance!

CPU: Intel E7500 @2.93
GPU: Asus Radeon HD 6700
Memory: Kingston 2GB DDR2 800mhz
Storage: WD Blue 1tb 7200rpm (it's the newest part of PC, bough it 3 months ago)
Motherboard: https://www.gigabyte.com/au/Motherboard/GA-EP45-UD3L-rev-11#ov
PSU: Blueberry PSB500 500W

It's a custom built PC, old around 9-10 years

Also when trying to boot, it beeps 3 times

It's a single stick

I've tried with cmos battery, same thing again.
I'll test the ram on friends computer tomorrow. Thanks a lot

I've tested ram on 2 different motherboards and it worked, after that I took ram from friends PC and placed it on my motherboard and it didn't work, so I'm 99% sure that ram slots are dead. Thanks for help, I though about buying new PC anyways and this just made that happen quicker. Cheers!