For Mean I would suggest using Col Moving Average
If(Modulo(Row(), 3) == 0,
Col Moving Average(:Col, 1, 2, 0),
.
)
jthi_0-1657882865907.png
Otherwise Data table subscripting might be helpful
If(Modulo(Row(), 3) == 0,
Mean(:Col[Index(Row() - 2, Row())])
)
-Jarmo