what pci slot do i have

I'm a novice and I am considering a RTX 3090. I am unsure what is meant when they say it is a triple slot card. Does that mean it is so wide that it needs to fit into 3 PCI slots? That would be extremely wide so I am confused... Or do you need to have a specific Mobo with slots near each...

What kind of PCI slot should I aim for and how many are usually needed?

So i bought a pre-built PC recently, that came with 4GB of RAM. I stupidly assumed that it would have 4 RAM slots, and was dissapointed when i opened it up. One of the sticks was occupied already (obviously) yet i still have the one free slot. Since i can't do Dual-channel, what is my best...

Perhaps I'm trying to push my mobo too hard, but I think I have read that PCI-e 3.0 is back compatible to previous PCI-e specifications.