If I understand what you are looking for the iif function may do it. Here is the basic idea - you will need to fill in the correct names and check syntax.
Select sum(iif(yourdate>Now()-90,amount,0)) as month3,
sum(iif(yourdate>Now()-180,amount,0)) as month6,
sum(iif(yourdate>Now()-360,amount,0)) as month12,
Item,
Location
From yourtable
Group by Item, Location
Where yourdate>Now()-160