My son is 3months, his polydactyly finger is swollen its black in color?

Your post lacks important data. Polydactyly can be as pronounced as a full normal extra digit or a thin dangling nubbin with minimal form/tissue. The minor ones often self destruct/self amputate when their blood supply does not adequately supply its needs. If you are worried about it contact his doc. When posting here the more data you provide the more we can help.

Your son must be seen soon. The discoloration may indicate tissue necrosis and/or infection. Please see the Emergency room right away. An imaging study may be indicated.