First, Toshiba had their own 3.5" drives long before the merger with Hitachi. For example, Toshiba MK2002TSKB 2TB 3.5" hard drive was on sale since 2011.
Toshiba's own design and factories wouldn't magically disappear after acquiring Hitachi's assets. So, after the merger, Toshiba sells some ex-Hitachi drives, for example toshiba DT01ACA300 3TB is obviously a relabeled Hitachi drive. But I believe that Toshiba MD03ACA300 3TB and Toshiba MD04ACA300 3TB drives are based on Toshiba's own design because they don't look like Hitachi or DT01ACA300. I suppose reliability and performance will be different between these three Toshiba 3TB drives. And it would be interesting to get some info on this.
Also, HGST is wholly owned by Western Digital. Despite regulators requirement, they didn't sell all 3.5" assets to Toshiba, only some of them. Most of the good stuff (that is, everything currently sold under HGST brand) still went to WD. So, even ex-Hitachi drives sold under Toshiba brand (by Toshiba) and under HGST brand (by WD) might have different reliability.