For CSGO, best you would need to run that somewhat well would only really be a 500$ build anyway, and I am pretty sure even some premades for 500$ would fit that easily. As for the other two games, you would need one hell of a good computer to play those properly while still retaining some visual aesthetics. I mean like, even my computer sometimes strains with Fallout 4 on the max settings, and it's about 1100$ of some of the best gaming parts you could get for the price range. (that was even with deals on some of the prices at some of the lowest prices they could be gotten for)
As for not knowing how to build one, it's either buy a premade (which would lower the quality of what you are paying for, trust me) or you can learn to put one together - that or ask a friend that does know to help you.