1. B
此题考查虚拟语气在条件状语从句中的应用.
这是对过去动作的虚拟,if从句用过去完成时,而主句用would have done sth做谓语.
此时可以把从句的助动词had提前至句首,并省略if.
即：Had I known it = If I had known it
2. D
此题考查强调句型.
其结构如下：It is/was + 被强调部分 + that + 句子未被强调部分.
其特点是：如果去掉it is/was和引导词that,剩下就是一个完整的句子（有时需要调整句序）.
则句子变为：The football match had to be put off because of bad weather.
此句完整,所以答案为D that.
3. C
固定句型hardly/scarcely...when...,意思是“一...,就...”.
其中前句/主句使用过去完成时,而when后面的从句使用一般过去时.
且主句如果把hardly/scarcely提前至句首,则主句采用部分倒装,即还要把助动词had（过去完成时）也提前.
即正常句序为：I had scarcely entered the room when the phone rang.
我一进入房间,电话就响了.
倒装成：Scarcely had I entered the room when the phone rang.
所以正确答案为C.