Your organization is deploying a single project for 3 separate departments.

Create a Shared VPC Host Project and the respective Service Projects for each of the 3 separate departments.

Create 3 separate VPCs, and use Cloud VPN to establish connectivity between the two appropriate VPCs.

Create 3 separate VPCs, and use VPC peering to establish connectivity between the two appropriate VPCs.

Create a single project, and deploy specific firewall rules. Use network tags to isolate access between the departments.