Is it uncommon for wisdom teeth to have 3 roots? Why do some teeth get three roots?

Not uncommon. Nearly all upper molars have the potential to grow three roots. Often the wisdom teeth have two or all three of the roots fused, but it isn't unocommon for the upper wisdom teeth to have three roots.

Maxillary molar teeth have three roots. Generally wisdom teeth will also have three roots, but sometimes may have two roots. Often times, the roots are dilacerate, or fused together. Mandibular molar teeth have two roots