Assuming you've created a function to hold the number selected via the Input Control entry field (week selected), just subtract three from this (for any periods covering the start of a year, you'll need to factor in how you deal with years and weeks unless more information is forthcoming).
Then, just apply a report filter to restrict the displayed weeks as between the selected week-3 and selected week.
For calculating the differences, take a look at the relativevalue() function.
HTH
NMG