Bronston Cafe

You can make Tier 3 dish by combining three of the same Tier 2 dishes. You can make Tier 4 dish by combining three of the same Tier 3 dishes. You can make Tier 5 dish by combining three of the same Tier 4 dishes. (Although, why would anyone ever do that?)
Tip: it is a good idea to buy pairs of dishes even if they are not in your Orders. If you get the third one, you can combine them into Tier 2 and sell it for profit.
Tip: if your Stock is full, you can still buy one dish if it could be immediately combined into a higher-tier one.