1.that → which
如果关系代词在定语从句中充当介词的宾语,而且介词和该关系代词一起提前至定语从句句首时,则指物时只能用which,指人时只能用whom.
该题which = the house,完整的定语从句为：
in which we once lived
= we once lived in which
= we once lived in the house
2.where → which/that 或 去掉where
如果定语从句缺少宾语时,则其需要用关系代词而不是关系副词来引导.如果定语从句中的动词缺少宾语,该关系代词可以省略.
关系代词which/that代替先行词the beautiful place,在定语从句we visited last year中充当visited的宾语,所以用关系代词which/that,而且可以省略.
即完整的定语从句为：
which/that we visited last year
= we visited which/that last year
= we visited the beautiful place last year
3.B
此题考查状语从句的省略语法,其中B if invited = if I am invited.
A错在if从句的时态不对；C错在用的是主动语态；D也错在从句时态.
该题时态根据状语从句的“主将从现”原则,要求if从句必须使用一般现在时.