I wouldn't say the Pirates are stacked at the position, but they have Hayes at 3B, Gonzalez currently at SS (although his future is probably at 2B), SS Llover Peguero is in AA and he was a top 100 guy not too long ago whose star has fallen somewhat. Marcano came over from the Padres in the Adam Frazier deal and he can play around the IF/OF. Surrendered Braves' international signing Jae-Hwan Bae is also in the IF mix for them. I don't know if Grissom would fit there, but he might.
Grissom has graduated out from prospect rankings, but I wonder what his value truly is. He's hit for average in Gwinnett with doubles power, but I think the question for him is will he hit enough to play 3B or improve his middle infield defense enough to play at a position where his bat will carry. He's still only 22 and he did get rushed last year, so his game is likely still in the refinement stage. I just wonder if his perceived ceiling isn't high enough to generate a lot of interest.
You forgot Oneil Cruz for the Pirates. He is their future at that position.