To find out how many cups of almonds Luis needs, we can set up a proportion based on the original recipe.
The recipe states that for every \( \frac{3}{4} \) cup of chocolate chips, \( \frac{1}{8} \) cup of almonds is needed.
First, let's find out how many groups of \( \frac{3}{4} \) are in 3 cups of chocolate chips:
\[ \text{Number of groups} = \frac{3 \text{ cups}}{\frac{3}{4} \text{ cups}} = 3 \div \frac{3}{4} = 3 \times \frac{4}{3} = 4 \]
This means there are 4 groups of \( \frac{3}{4} \) cups of chocolate chips in 3 cups.
Now, for each of these groups, \( \frac{1}{8} \) cup of almonds is needed. Therefore, we will multiply the number of groups by the amount of almonds needed per group:
\[ \text{Total almonds} = 4 \times \frac{1}{8} = \frac{4}{8} = \frac{1}{2} \]
Thus, Luis will need \( \frac{1}{2} \) cup of almonds.
Rounding to the nearest tenth, \( \frac{1}{2} \) cup is 0.5 cup.
Therefore, the answer is 0.5 cup.