At the TB level, the difference is closer to 10%.

Three binary terabytes i.e. 3 * 2^40 is 3298534883328, or 298534883328 more bytes than 3 decimal terabytes. The latter is 298.5 decimal gigabytes, or 278 binary gigabytes.

Indeed, early hard drives had slightly more than even the binary size --- the famous 10MB IBM disk, for example, had 10653696 bytes, which was 167936 bytes more than 10MB --- more than an entire 160KB floppy's worth of data.