How do we write years before AD 1000?

For years with 4 digits, usually we write it this way:

George was born in the year 1732.

or

George was born in 1732.

What about years with three, two, or even one digit?

Are these examples still valid?

1. George was born in the year 732.

2. George was born in 732.

3. George was born in the year 32.

4. George was born in 32.

5. George was born in the year 2.

6. George was born in 2.

All your examples are all acceptable: it is not normal practice to use leading zeros.

I would, however, suggest prefixing (or suffixing) all of them with AD before (or after) the digits (especially so where you omit "the year"). I believe (but have not confirmed) that it is conventional to include "AD" for years of less than 4 digits (unless it is obvious from the context what era you are discussing), and the presence of "AD" helps to indicate that you have not unintentionally omitted a digit from the year. It also, incidentally, clarifies (i) which calendar system you are using, and (ii) that you do not mean BC.