yes, you read 中 as ちゅう , and 中3 as ちゅうさん
中3 is a shorthand way of writing 中学校3年生 which is (Japanese) Junior High School 3rd graders or in other words, 9th graders.
so 中3の25% would be "25% of (Japanese) 9th graders"
so the full translation of
「中3の25%、短文も理解困難」
is
"Twenty-five percent of 9th graders have difficulty understanding short passages."
Thanks to @Mars and @Eiríkr Útlendi for suggesting the more accurate word choices.
After all this editing, I realized one final point that can be made: in the expression 短文も理解困難, も carries the meaning "even", so the most apt translation would be:
"Twenty-five percent of 9th graders have difficulty understanding even short passages."
what in America would be "highschool freshmen"is basically inaccurate except with respect to which year of schooling counting from the bottom. JHS != HS. – virmaior Dec 01 '18 at 06:43