"three by five"doesn't make sense in chinese, you need to consider the context, that is, what does this "three by five" indicate to. so here it shoule be "三英寸宽，五英寸长"

可以简单的说：“三乘五”的卡片，在中国这个长度单位一般指的是厘米，如果不是厘米可以加上个其它的单位，例如：“三乘五英寸”

一张三英寸宽，五英寸长的卡片

It's pronounced "three by five". how do you say it in chinese?

A 3x5 is an index card measured in INCHES. A common use for them is for flashcards when studying. Yes there are bigger index cards these days. 4x6 and maybe larger, but the 3x5 from my memory was the traditional one and its measured in inches.