linear regression of a non-linear dataset

Hello,

I want to use a linear regression for every three datas of my given dataset x1. I have a 3x480 matrix full of this data in the x and the y dimension. I was thinking of fitting every collom (3 datapoints) once and put the result of my magnitude into an array. I tried it using this loop:

U=double.empty(3,(length(x1)),0);
for u=1:1:length(x1)
  uu = fitlm(x1(1:3,u),y1(1:3,u));
  U(1:3,u)=uu.Fitted(1:3,1);
end

But it gives the Error

Attempt to grow array along ambiguous dimension.

Error in polyfit_Inet_v4 (line 26)

What am I doing wrong? Thank you for your help!