Much more efficient and actually yielding all possible combinations:
N = 101;
x = linspace(0,0.5,N);
%-------------------------------------------------
%This bit here is more efficient than the one below. Loops can be your friend.
tic
numVals = numel(x);
total_elements = sum(1:numel(x));
results = ones(total_elements,3);
pos = 1;
for ii = 1:numVals;
idx = pos + numVals - ii;
results(pos : idx, 1) = x(end - ii + 1:-1:1);
results(pos : idx, 2) = x(1:end - ii + 1);
results(pos : idx, 3) = x(ii);
pos = idx + 1;
end
toc
%---------------------------------------------------
%KSSV's answer:
tic
y = linspace(0,0.5,N) ;
z = linspace(0,0.5,N) ;
[X,Y,Z] = ndgrid(x,y,z) ;
x = X(:) ; y = Y(:) ; z = Z(:) ;
thesum = x+y+z ;
% get the sum 0.5
idx = find(thesum==0.5) ;
iwant = [x(idx) y(idx) z(idx)];
toc
