What is the difference between `Common`, `RecordInsertList`, and a normal table buffer?

They solve different problems.

A normal table buffer is strongly typed to one table:

CustTable custTable;

Common lets generic code hold different table buffer types:

Common common = custTable;

RecordInsertList is a helper for efficiently collecting and inserting multiple records.

So I would not think of these as three versions of the same thing. One is a normal record buffer, one is a generic record buffer, and one is a bulk insert utility.