Slot 3 is a PCIe ×16 slot which is recommended for NA255A in support of the external GPUs.

Slot 6 and Slot 7 are PCIe ×8 slots which are recommended for the NS780S-G3.

Mac Pro 2019 has eight PCIe x16* 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 NA255A 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) [recommended for NS780S-G3]
• Slot 7: up to x8 lanes (single-wide) [recommended for NS780S-G3]
• 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