On pumping stations (workbench, oil platform, refinery) there were extra ports added a while ago. However the third port doesn't affect the total flow rate. With 2 hoses you get 50L/s and 45L/s and with 3 hoses you get 32L/s, 36L/s and 27L/s (or about that much)

Tested with 3 large pumps pumping both in and out, on multiple vehicles. Should be easy to reproduce:

• get a tanker with enough volume
• Spawn it at an oil platform
• Make sure that infinite resources are not on when pumping (not sure if this is necessary)
• connect al hoses and turn on pumps on both ends.