水平对齐的图像具有相等的宽度和静态边距？

I'm trying to create a responsive image gallery for a project I'm working on and I've run into some trouble. For the mobile version of the site, I want each row of the gallery to have three thumbnails of equal width and a 15px gutter. I can get the images to take up equal space by making them each 33.33% wide, of course, but the 15px right margin I attempted to apply to the first two images in the row pushed the third off. Any solutions?I'm trying to create a responsive image gallery