Nested unbalanced regression analysis

Hello, I am having a problem figuring out how to model a continuous  
outcome
(y) given a continuous predictor (x1) and two levels of nested  
categorical
predictors (x3 nested in x2). The data are observational, not from a
designed experiment. There are about 15 levels of x2 and between 3  
and 14
levels of x3 nested within each level of x2. There are between 6 and  
50 x1
and y observations for each unique x3 (i.e. the data are  
unbalanced). I
would like to get a prediction equation for y based on x1 and the  
levels of
x2 and x3, and be able to test for significant effects of the levels  
of x2
and x3. The variables x2 and x3 are drawn from a fixed population.   
I deeply
appreciate your assistance if available to assist.  R/Bob




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