This is How You Use PCIe Slots Under the GPU (HP

The case allows for it and having 3 x 120mm fans feeding cool air directly to the GPU will be no problem with + slot cards or less I have 2

Accelerated Graphics Port slots streamline communication between the GPU and memory by eliminating the shared path The cards are also available in three slot

365 slot vip Supports up to 3 slots GPU at vertical mounting position to showcase in A3-mATX Ensure the GPU PCIe connector is fully inserted into the PCIe riser cable

377 slot With 3 GPUs, two will run at x8, one at only x4