Not sure if you got this sorted out or not, but since I just did the same thing today I thought I’d share the solution.
The shortcode on your page that you would need for 1 row 3 columns would be:
[woothemes_testimonials limit="3" per_row="3"]
If you’re not using a theme from WooThemes simply adding the above shortcode won’t generate the column styles since Testimonials by WooThemes comes styleless.
Try adding the following to your CSS for the columns (courtesy of WooThemes Canvas theme). Keep in mind that the following CSS might require tweaking but covers up to 6 columns:
.widget_woothemes_testimonials .quote{float:left;margin-right:5%}
.widget_woothemes_testimonials .quote.last{margin-right:0}
.widget_woothemes_testimonials .quote img{float:left;margin:0 1em 0 0}
.widget_woothemes_testimonials .columns-1 .quote{width:100%}
.widget_woothemes_testimonials .columns-2 .quote{width:47%}
.widget_woothemes_testimonials .columns-3 .quote{width:30%}
.widget_woothemes_testimonials .columns-4 .quote{width:21%}
.widget_woothemes_testimonials .columns-4 img{float:none;margin:0 auto;display:block}
.widget_woothemes_testimonials .columns-4 .author{display:block;text-align:center;float:none}
.widget_woothemes_testimonials .columns-5 .quote{width:16%}
.widget_woothemes_testimonials .columns-5 img{float:none;margin:0 auto;display:block}
.widget_woothemes_testimonials .columns-5 .author{display:block;text-align:center;float:none}
.widget_woothemes_testimonials .columns-6 .quote{width:12.4%}
.widget_woothemes_testimonials .columns-6 img{float:none;margin:0 auto;display:block}
.widget_woothemes_testimonials .columns-6 .author{display:block;text-align:center;float:none}