I don't fully understand.
The only way I see feasible to make a statement about the biological processes associated to a group of proteins, being a small group or a big group, is by using the information of the GO annotations. If the group is big, you need the help of an enrichment analysis or Fischer test
But if you work with a small group or proteins as is your case, then it has to be ease for you to figure out how similar are their biological function without having to run a comparison or a complex task
If you see that all of the GO are similar, you only get a conclusion.
If they are different, you get another different conclusion
That being said it is important to say that you need to know what are you comparing.
GO annotations navigate among different levels. Then, you can have many proteins associated to a same GO level (e.g, proteins corresponding to the general nitrogen metabolism (Level2) ) when they are actually different proteins that are part of this level, such as nitrogenase, nitrate reductase, nitrite reductase, and so on (that is levels 3 or 4)