For the exponential decay function \( y = 3(0.75)^x \), as \( x \) approaches infinity, the term \( (0.75)^x \) approaches 0. Therefore, the function approaches a value of \( y = 0 \) as \( x \) becomes very large.
The horizontal asymptote of the function is thus:
\[ y = 0 \]
So, the answer is:
The horizontal asymptote is at \( y = 0 \).