While I largely agree with @Erica's answer, I would suggest going about it slightly differently. Personally, I would suggest adding a new text field called, for example, "Category". Then, use the field calculator geoprocessing tool or right click on that new field in the attribute table and bring up the field calculator window. (http://resources.arcgis.com/en/help/main/10.1/index.html#//005s00000029000000) Then, as the expression for calculating the value of the new field, you would simply use
!FieldName! as the calculate expression, replacing FieldName with whatever the field name is for the field that currently has the values like 'HfD2' in it. And make sure the parser in field calculator is set to Pyton.
Using that expression should populate your new field with the A, B, C.... as appropriate, assuming what you said about that always being the 3rd character is correct. And by the way, in case you are not familiar with Python, yes,  in the calculate expression is not a miss-type, Python starts counting with 0, so 2 is actually the 3rd character.
Then, once you have a field with the categories in it, you can open up the layer's properties in ArcMap and click on the Symbology Tab > Categories > Unique Values. Then, set the Value field to the new Categories field you just created and populated, and click the "Add All Values" button. Then click on the individual symbols and change colors/symbols as appropriate.