Slot 3 is a PCIe ×16 slot which is recommended for NA265A in support of the external GPUs.

Rack Mount Mac Pro 2019 has eight PCIe ×16* sized slots that accept many different types of PCIe cards:

• Slot 1: up to x16 lanes (double-wide)
• Slot 2: up to x8 lanes (double-wide)
• Slot 3: up to x16 lanes (double-wide) [recommended for NA265A for external GPUs]
• Slot 4: up to x16 lanes (double-wide)
• Slot 5: up to x16 lanes (single-wide)
• Slot 6: up to x8 lanes (single-wide)
• Slot 7: up to x8 lanes (single-wide)
• Slot 8: up to x4 lanes (half-length)

For more information about slot layout of Mac Pro 2019, please visit:
https://support.apple.com/en-us/HT210104