I am conducting a systematic review about a community-based intervention to prevent hospitalizations in older adults. We identified 3 studies that reported on the outcome of interest and I am not sure whether I can pool their results in a meta-analysis. The first study reported data on the comparison between community A and community B for year 2006. The second study reported data on the comparison between the same community A and a group of 5 surrounding communities (community B was not included in that comparison) for years 2006, 2007 and 2008. The third study reported data on the comparison between communities C and D for the year 2006.
My understanding is that it would be appropriate to pool the results of the 3 studies for all the years because ultimately they report effect estimates concerning different sets of comparisons, even if community A was part of those comparisons for a specific year in the first and second study. Is that understanding correct? Would someone have any reference supporting or refuting that approach?