A baker made 18 cupcakes. The first row has 2/3 of the total cupcakes, and the second row has 1/4 of the total cupcakes. How many cupcakes are in the third row?

The first row has $\frac{2}{3}$ of the total 18 cupcakes.

$$\text{Cupcakes in first row} = \frac{2}{3} \times 18$$ $$\text{Cupcakes in first row} = 2 \times \frac{18}{3}$$ $$\text{Cupcakes in first row} = 2 \times 6$$ $$\text{Cupcakes in first row} = 12$$

The second row has $\frac{1}{4}$ of the total 18 cupcakes.

$$\text{Cupcakes in second row} = \frac{1}{4} \times 18$$ $$\text{Cupcakes in second row} = \frac{18}{4}$$ $$\text{Cupcakes in second row} = 4.5$$

Since cupcakes cannot be in halves, there might be an issue with the problem statement or an assumption that the number of cupcakes must be whole. However, following the calculation directly:

The total number of cupcakes is 18. The number of cupcakes in the third row is the total minus the cupcakes in the first and second rows.

$$\text{Cupcakes in third row} = \text{Total cupcakes} - \text{Cupcakes in first row} - \text{Cupcakes in second row}$$ $$\text{Cupcakes in third row} = 18 - 12 - 4.5$$ $$\text{Cupcakes in third row} = 6 - 4.5$$ $$\text{Cupcakes in third row} = 1.5$$

The number of cupcakes in each row are:
First row: $\boxed{\text{12 cupcakes}}$
Second row: $\boxed{\text{4.5 cupcakes}}$
Third row: $\boxed{\text{1.5 cupcakes}}$