Why does my (pretty good) PC not run any game well?

My PC, which is pretty good, can't run Counter-Strike Global offensive higher than 30 FPS, Battlefield 3 runs okay-ish, Even Minecraft runs at about 100 FPS max, while I feel this should be a lot higher. But it is just having a hard time with graphics. This surprises me, because the specs are as followed:

AMD FX-8320 8-core CPU (3.50 GHz/8MB Cache/AM3+)
Gigabyte 970A-DS3P AM3+ Motherboard
2 x8GB KINGSTON Hyper-X Fury Dual-DDR3 1600 MHz
3GB AMD Radeon R9 280 Graphics Card
Corsair H55 Cooler

It is properly cooled; it has 4 fans, plenty of power.
I'm, however, running 3 monitors, 2 of which on a separate, old graphics card, an nVidia Quatro.

AMD says they have no new drivers available, so old drivers are not the case.

Thanks in advance, please let me know if you want any more details.

Added (1). I have a 120GB SSD C-drive, of which half is available, and a 500GB HDD, 60% available.

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