masterinex

Hi guys , say I have 3 random variables X1,X2 , X3 where X1 = Y1 + Y2 + e1 X2 = Y2 + Y3 + e2 X3 = Y1 + Y1 + e3 where X1,X2,X3 are normally distributed with N(0,1) and e1,e2,e3 are independent random variables (error terms) with normal distribution Do I have to include the error terms for the mean vector of X and the covariance maxtrix of X , if so how would it look like ? mean: mean(X) = [mean(X1)mean,(X2),mean(X3)]' much thanks