Looking at this question, first, I thought for a while about redundancy. A triangular prism. I searched on Wikipedia for the terms prism that says At least two of the flat surfaces must have an angle between them. (means triangle, isn't it? and triangular prism that says a three-sided prism.
In addition, I searched Google Images with triangle prism and prism. Most of them were triangle!
Of course, the latter one talks about geometry but still, a triangular prism? Why simply not say prism and it includes its shape i.e. triangle! Fun is, I searched for round prism and rectangle prism - both have results! Google is great!