3-tuple NAT allows external devices to proactively access internal PCs through translated addresses and ports.

Explanation From HCIA-Security documents:

3-tuple NAT is used to publish internal services to external users by translating destination IP address, destination port, and protocol so that an Internet user can reach an intranet server using a public address and specific service port. So the first part of the statement about enabling proactive external access through translated addresses and ports matches what 3-tuple NAT is used for.

However, NAT translation and firewall access control are two different functions. NAT only defines how addresses and ports are translated; it does not replace the firewall’s security policy decision. On a Huawei firewall, whether a packet is allowed to pass is determined by the configured security policy (interzone policy). If no relevant security policy exists to permit the traffic from the external zone to the internal zone and the mapped service, the firewall will not automatically allow it just because a NAT mapping is present.

Therefore, without an appropriate permit security policy, the access packets will be denied even if 3-tuple NAT is configured.