Something like this
% dummy data
N = 15;
Range1peaks = [10*rand(N,1)-70 2*rand(N,1)+4 randi([0 5],N,1)]
% sort
[~,Peaks_index1] = sort(Range1peaks(:,3),1,'ascend');
Arg_Range1peaks = Range1peaks(Peaks_index1,:);
% append column, take mean, fill
Arg_Range1peaks = [Arg_Range1peaks zeros(size(Arg_Range1peaks,1),1)];
Sputum_Idx = unique(Arg_Range1peaks(:,3));
for i = 1:numel(Sputum_Idx)
ind=(Arg_Range1peaks(:,3)==Sputum_Idx(i));
Arg_Range1peaks(ind,4) = mean(Arg_Range1peaks(ind,1));
end
% show result
Arg_Range1peaks