(The PCI Express x16 slot conforms to PCI Express 3.0 standard) HELP!!

Solution
The pcie 16x is backwards compatible. So Pcie 3.0 will work with all pervious standards. To answer your questions, ALL them will work (with some minor expections).
The PCI Express standard keeps upgrading every few years. If you have a motherboard from 2007/2008, it'll have the PCI-E 1.1 standard, and one slot that supports PCI-E 1.1 x16 cards.

As newer standards come, performance increases. 1.0a came first, then 1.1, 2.0, 2.1, 3.0. All slots look the same, physically.

Some AMD cards didn't support 1.0 or 1.1, only 2.0 and above. If you put such a card in a 1.1 slot, the system wouldn't boot. Such cards were called 2.1-compatible, and they worked with 2.0, but not older.

The newer slot you have, the better you'll support newer graphics cards when they come out.