Correct Solution One: In the above methods, excluding the method of planting only 2 types (choosing any 2 out of 3 crops $\mathrm{C}_{3}^{2}$, and each choice has 2 methods, i.e., $2 \times \mathrm{C}_{3}^{2}=6$, such as ababa or babab), there are 48-6=42 planting methods.

Correct Solution Two: First plant $\mathrm{A}$ and $\mathrm{B}$, there are $3 \times 2$ ways. If $\mathrm{C}$ is the same as $\mathrm{A}$ and $\mathrm{D}$ is the same as $\mathrm{B}$, then there is $1 \times 1 \times 1=1$ way; If $\mathrm{C}$ is the same as $\mathrm{A}$ but $\mathrm{D}$ is different from $\mathrm{B}$, then there are $1 \times 1 \times 2=2$ ways; If $\mathrm{C}$ is different from $\mathrm{A}$ and $\mathrm{D}$ is different from $\mathrm{B}$, then there are $1 \times 2 \times 2=4$ ways. In total, there are $3 \times 2 \times (1+2+4)=42$ planting methods.
Example 4. Plant 3 types of crops in 5 experimental fields as shown in the figure, with each field planting one type of crop and adjacent fields not planting the same type of crop. How many different planting methods are there?