I actually asked an inorganic chemist the very same question, and this is what he told me:
I seperated out the section that I thought was the best reason he gave, and I think it's important to read the last paragraph. His other reasoning seems good, too. I'm not trying to say it doesn't make sense. I just think the part I seperate is probably what they had in mind when designating the groups.
When you say they are put into the same "group" I assume you mean the group B numbering within the transition metals columns, which would make the Fe, Co and Ni triads all part of group VIIIB. Now with the main group, also termed group A, the significance of the roman numeral to signify the standard oxidation state of the element is obvious. The oxidation states get more complex in the transition metal series and frequently multiple oxidation states are observed. The Cu triad (Cu, Ag, Au) is termed IB due to stable +1 states that are common especially silver which is only found as elemental or +1, where copper may alos be found as a +2 and Au may be found as +3. The Zn triad (Zn, Cd, Hg) are exclusively +2 and are thus labled IIB group. You then skip back to the begining of the transition metals and number across due to stable +3 and +4 for Sc triad and Ti triad respectively. There are a few reasons why in the classical B grouping Fe, Co, and Ni are lumped into a "group" in the middle of the transition metal series. First as you go across the transition metals the +2 state is becomeing continuously more stable and prevalent. For Fe there are several oxidations states that are common, however +2 and +3 are most stable and common. When you move over to Co the +2 becomes the dominant oxidation state with +3 also commmon and very rare examples of higher oxidation states. When you move over to Ni it is almost exclusively +2 or +3 with rare exceptions. The second point that I would consider in grouping them together would be their adherence to the 18 elecron rule. These elements in the middle of the transition metall series follow the rule almost always where earlier elements frequently violate the rule with lower electron counts or even odd number counts and the later elemnts are frequently content with 16 or even 14 electron counts, the later condition is common for group IIB.
Another consideration to give this group the lable VIIIB is the d8 configuration that is common (Fe(0), Co(I), and Ni(II)) are all d8 systems and frequently analogous compounds can be made with the different metals.
I hope this shed some light on your question. I also should note that the system that groups these elements together is an older group numbering system and the new and current grouping system avoids the grouping by groups A and B and simply numbers across the table from 1 to 18, with groups 3-12 being the transition metals.
Chris Potratz -University of Ohio