I understand those are the equations to be estimated; however, those equations can only be estimated simultaneously if the data used to estimate each equation are linearly independent of the other two equations.
If you would like to estimate each equation independently (or any two out of the three independently of the third) using the HAVE data set that you provided you would need to specify two (or three) FIT statements in PROC MODEL:
proc model;
y1=a+b*x;
y2=c+d*x; y3=e+f*x; fit y1 / gmm kernel=(bart,2,0) vardef=n;;
fit y2 / gmm kernel=(bart,2,0) vardef=n;
fit y3 / gmm kernel=(bart,2,0) vardef=n;
run;
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