三者是等价的.
只有在一些特殊情况下,才不行.
(1) 如果先行词是all,much,anything,something,nothing,everything,little,none等不定代词,关系代词一般只用that,不用which.例如：
All the people that are present burst into tears.
(2) 如果先等词被形容词最高级以及first,last,any,only,few,mush,no,some,very等词修饰,关系代词常用that,不用which,who,或whom.
(3) 非限制性定语从句中,不能用关系代词that,作宾语用的关系代词也不能省略.例如：
There are about seven million people taking part in the election,most of whom、are well educated.
如果作先作词的集体名词着眼于集体的整体,关系代词用which;若是指集体中的各个成员,则用who.