If you need 3 of one level of item to obtain one item of the next level, then the total number of level 1 items to obtain an item of level x would be 3^(x-1). So a level 2 item would take 3^1 = 3 level one items, a level 3 item would take 3^2 = 9 items, a level 4 3^3 = 27 items, so on and so forth until we reach a level 15 item taking 3^14 = 4,782,969 level 1 items.

The answer is 4,782,969 level one items to make a level 15 item.

I really hope you can gather level 1 items very quickly, or this may take a while.