As shown in the image below, it is possible for 3-legged tantalums to have two types of polarity.
On the IBM 51xx motherboards and cards, IBM have used tantalums of the negative-positive-negative polarity (outer legs are negative).
Option 1 of 2: Replacing 3-legged with 3-legged
When you replace a faulty 3-legged tantalum with a 3-legged replacement, the replacement must be of the same polarity.
On the IBM 51xx motherboards and cards, the 3-legged tantalums are of the negative-positive-negative polarity, and of 10µF/16V. They typically have "106 16V" or "10µF 16V" marked on them.
Once you have a suitable 3-legged replacement (of capacitance/voltage/polarity), it can be soldered on to the PCB in either orientation (lettering to the front, or lettering to the back). That is because the same polarity relationship is maintained - positive leg into positive hole, and negative leg into negative hole.
The diagram below is for a particular negative-positive-negative 10µF/16V 3-legged tantalum made by Kemet, and available at Mouser Electronics:
Mouser part number: 80-T398E106M16AT
Kemet part number: T398E106M016AT
Option 2 of 2: Replacing 3-legged with 2-legged
On the IBM 51xx motherboards and cards, there is no reason why you cannot replace a 3-legged tantalum with a 2-legged one.
Ensure that you observe correct polarity - positive leg into positive hole and negative leg into negative hole.