Using PBSPLINE in the SGPLOT might lead to unwanted wiggles. Another way to visualize is to score the model on a grid of x values:

data new; /* scoring data set */
y=.;
do trt=1 to 3;
do x=0 to 10 by 0.1;
output;
end;
end;
run;

...

proc sort data=anew; by trt x; run;

proc sgplot data=anew;
scatter y=y x=x /group=trt;
series y=predy x=x /group=trt;
run;