Multiple buildings

What my understanding is, is that you would use BD+C Master Campus approach, not Group approach. With Master Campus, each of the 3 buildings shares a campus and therefore shares the LT and at least some if not all of the SS credits. Each building has a separate certification and requires a separate name and designation for that purpose. For Group, its required that each of the buildings is substantially similar; for instance I am working on an apartment complex with two buildings which only have a few ID changes in lower floors but are otherwise the same, and a third building which is very similar but slightly bigger, all with the same orientation and sharing a basement. To be fair, the distinction between Group and Master Campus is not that well made and you probably want to discuss with someone at GBCI, but I would assume for your projects that some buildings carry out different functions and therefore will be architecturally distinct, not just a row of similar sheds. I believe you would be looking at Master Campus. 

The part where you describe the building completion dates spanning three years would indicate that it may not be desirable to certify as a single "group" project, even if you were eligible to do so. 

Here is the main page to help you evaluate the potential paths: https://www.usgbc.org/sites/default/files/LEED%20Campus%20Guidance_4.1.2014(6).pdf

I think Melissa is on the right approach, Group approach must all be constructed at the same time, under a common contract - an example might be two or three dormitory buildings built at the same time, through one contract, and that have the same technical requirements, layout, use, etc.

Campus may benefit your client, or they may decide to pursue each building's certification separately if the project teams will be different, different requirements, unknown timeline, etc.