As a workaround, you could just add the last 3 periods together in a running total and then divide by 3.
Something like:
Dim Period1 As Decimal = api.data.GetDataCell("Cb#XXXXXX:T#Pov:A#Direct_HC:S#Actual:E#XXXXX:F#EndBal_Input:V#Periodic:O#BeforeAdj:I#None:U3#Top:U4#Top:U5#Total_Adj_Alloc:U2#" & U2 & ":U7#Top:U8#None").CellAmount
Dim Period2 As Decimal = api.data.GetDataCell("Cb#XXXXXX:T#PovPrior1:A#Direct_HC:S#Actual:E#XXXXX:F#EndBal_Input:V#Periodic:O#BeforeAdj:I#None:U3#Top:U4#Top:U5#Total_Adj_Alloc:U2#" & U2 & ":U7#Top:U8#None").CellAmount
Dim Period3 As Decimal = api.data.GetDataCell("Cb#XXXXXX:T#PovPrior2:A#Direct_HC:S#Actual:E#XXXXX:F#EndBal_Input:V#Periodic:O#BeforeAdj:I#None:U3#Top:U4#Top:U5#Total_Adj_Alloc:U2#" & U2 & ":U7#Top:U8#None").CellAmount
Dim Avg As Decimal = (Period1 + Period2 + Period3)/3
