No, CD-RW is not the same as Combo.
CD-RW drives write CD-R and CD-RW, and read CD-ROM, CD-R, and CD-RW.
Combo drives do all the above, plus add the ability to read DVDs.
DVD-RW drives do all the above, plus add the ability to write DVD-R and DVD-RW.
No G3 iMacs ever had combo drives out of the box. If you ever see that, it's something someone upgraded after purchase.
tooki