You had allocated an empty vector and were growing it by concatenation. It works, but is often slower. Since you know the size of the vectors, just preallocate them at their final size and assign the values by indexing.
% Calculation of rotation matrixes
npos = numel(ERA);
GCRS_X = zeros(npos,1);
GCRS_Y = zeros(npos,1);
GCRS_Z = zeros(npos,1);
for i=1:npos
R= Rotation_3(-ERA(i));
R2= Rotation_2((Xp(i)./3600) *pi/180);% conversion of arcsecond to radians
R1= Rotation_1((Yp(i)./3600) *pi/180);% conversion of arcsecond to radians
W = Rotation_3(-((s(i)./3.6e9)*pi/180))*R2*R1;
Trs= R*W;
% Transformation of ITRS positions to inertial GCRS positions
GCRS_X(i) = Trs(1,1)*x(i)+Trs(1,2)*y(i)+Trs(1,3)*z(i); %GCRS_X in Meter
GCRS_Y(i) = Trs(2,1)*x(i)+Trs(2,2)*y(i)+Trs(2,3)*z(i); %GCRS_Y in Meter
GCRS_Z(i) = Trs(3,1)*x(i)+Trs(3,2)*y(i)+Trs(3,3)*z(i); %GCRS_X in Meter
end
