It's easy if you use FormCalc.
What you need to do is rename all your rows to the same thing (so say Row1) and then they will automatically become Row1[0],Row1[1]. etc. Then all you do is in the exit event of the dropdown list put in something like:
if ($ == 1) then
Table1.HeaderRow.Gold.presence = "hidden"
Table1.HeaderRow.Silver.presence = "hidden"
Table1.Row1[*].Gold.presence = "hidden"
Table1.Row1[*].Silver.presence = "hidden"
elseif ($ == 2) then
Table1.HeaderRow.Bronze.presence = "hidden"
Table1.HeaderRow.Silver.presence = "hidden"
Table1.Row1[*].Bronze.presence = "hidden"
Table1.Row1[*].Silver.presence = "hidden"
elseif ($ == 3) then
Table1.HeaderRow.Gold.presence = "hidden"
Table1.HeaderRow.Bronze.presence = "hidden"
Table1.Row1[*].Gold.presence = "hidden"
Table1.Row1[*].Bronze.presence = "hidden"
endif
just make sure you have 'specify item values' ticked in the binding tab of the dropdown list.
You probably want to also put Table1.HeaderRow.Bronze.presence = "visible" etc back into the other choices in case the person changes their mind.