Ok from the manual for the board it looks like your slots are like this
Starting at the top
1- x16 - Install the video card here
2- x1 slot(covered by your video card in almost all cases)
3- x16 slot that only runs at x1 speed(install your x1 card here if you wish)
3- x16 slot that only runs at x4 speed(I recommend installing your card here because it will give the video card more space to get air)
2.6 Expansion Slots (PCI Express Slots)
There are 4 PCI Express slots on this motherboard.
PCIE1 (PCIE 3.0 x16 slot) is used for PCI Express x16 lane width
graphics cards, or to install PCI Express graphics cards to support
PCIE2 (PCIE 2.0 x1 slot) is used for a PCI Express x1 lane width card,
such as a Gigabit LAN card or SATA2 card, etc.
PCIE3 (PCIE 2.0 x16 slot) is used for PCI Express x1 lane width graph
PCIE4 (PCIE 2.0 x16 slot) is used for PCI Express x4 lane width graph
ics cards, or to install PCI Express graphics cards to support CrossFi
function. And for installing ASRock Game Blaster.