I want to build a model and I currently have the equation: Y = x1 + x2 + x3 + u1

I then have explanatory variables for each of the variables on the right
So:
x1= p1+p2+p3+u2
x2= s1+s2+s3+s2
x3= a1+a2+a3+u2

Which model will allow me to ensure that all of these variables are considered when I am creating the model? I thought about using a simultaneous equation model but my initial independent variable Y does not occur in the later equations so I do not think I can or I do not know how to use this model.

Any tips or advice is greatly appreciated!