data = readmatrix('table.xlsx');
time = data(:,1);
PW = data(:,2);
You can identify all the indexes for which the difference between two rows is less then three in the following way.
idx = abs(diff(time)) < 3;
However, idx now has length(time)-1 elements (because it perofrms subraction of i+1 and i elements one pair at a time).
You can easily fix it with
idx = [idx(1);idx];
And then plot your selected data
scatter(time(idx),PW(idx),[],PW(idx),"filled")
