Wayne - Yes, if you can Rank all the rows/records in your data (and vertically repeated time series observations over time will always rank naturally by time), then you can refer to prior/past/higher/lower records using subset functions that reference ranked values plus or minus a fixed number of rows. In this case the 3-month arithmetic Formula is:

IF([Row Rank]< 3,0,

(SUBSET_SUM([Monthly Values], SUBSET([Row Rank],([Row Rank]-2)))
+
SUBSET_SUM([Monthly Values], SUBSET([Row Rank],([Row Rank]-1)))
+
SUBSET_SUM([Monthly Values], SUBSET([Row Rank],([Row Rank]-0)))

)/3

)

I attach an example file. Note that you do not need this formula explicitly in the file data set to graph moving averages, the Graph View has this option built-in, as shown in the example file.