so i did a little investigating.. as i do... and i think I've figured out whats in 2 of the boxes. still cant figure out the 3rd. so here's what i think is in them (not a fact, just a guess) but i think you will agree that they would fit perfectly into the silhouettes.
For buying all 3 boxes (300AG) you get a bonus item, which is the big #1 candle.
Big #1 Candle
Strawberry Cake Cherry cake Another cake? Bonus Item
All of the ones i could figure out can be seen in the anniversary park
I'm also guessing the cakes are in decor. so don't worry about wasting your gold on a consumable item.
UPDATE: after looking at the pico page again, I cannot decide if the "complete item" refers to the bonus candle for buying all the boxes, or if you get another bonus once all the boxes are open and you have all 3 items.
AND AGAIN. I REPEAT. THIS IS ALL GUESSES.