It's been a long time since school..
As an engineer I feel like I'm putting my head on the block here, but it's been a while since I interpolated between values, even with two variables. And when a situation at work demanded linear interpolation with three variables, I must admit that my mathematical memory failed me..
But why think long and hard, when you can solve it with geometry and Dynamo?
Case: I was given four allowed ground pressure values by a geotechnician by two variables: Foundation depth and foundation base width
So three variables can be represented in 3d-space: X=Base width, Y=Depth and Z=Allowed ground pressure. And if the values are linear they should form a plane, right?
Now if I create a X-coordinate and an Y-coordinate (My wanted values) and project it vertically onto the plane it gives me a point which represents an interpolated value of all the three variables.
Extract the Z-Value and you got your allowed ground pressure!
the simple definition:
So 177.5 it is.. no sweat.