There are many ways to do this, here is one option:
contents = fileread('results.txt'); %read whole file at once
contents = strsplit(contents, 'Particle'); %split into cell array at particle
particles = cellfun(@(c) sscanf(c, '%f', [3 Inf])', contents(2:end), 'UniformOutput', false); %scan each section into a 3xN matrix
