在意义上：它们都可以有“只要”的意思
区别：
so long as:1.只要.eg:He need not step down so long as they support him.只要他们支持他,他就不用辞职.
2.既然,由于.
as long as:1.达...之久
2.=so long as
so far as:=as far as
1.到...那么远；远到,一直到
2.(表程度、范围）就...;尽...;至于
这个后面一般可接：
as far as I know:据我所知
as far as possible:尽可能,尽量