首先先来看一下这三个词的英文解释
countless
adj.too numerous to be counted
例句:countless hours
numerous
adj.
1.[修饰复数名词]许多的,大量的2.[修饰单数名词]为数众多的
adj.amounting to a large indefinite number
例句:numerous times
the family was numerous
innumerable
adj.too numerous to be counted
例句:innumerable difficulties
从上面可以看出 innumerable =countless 意为 数不尽的,无数的.
而numerous是修饰复数名词的,指的是许多的,大量的