Average size delivered per person for the last 3 sprints

[SquidgyWidget]

I need a measure to calulate the average size of cards delivered per person for the last 3 sprints to assist with Agile sprint planning. Sample data might look like below: 

The results from below meaure does not correctly calculate. 

_AvgPointsDelivered = 
    var _Last3Sprints =
        SUMMARIZE(
            FILTER(
                SUMMARIZE(
                    'Sprint Snapshots',
                    'Sprint Snapshots'[Sprint],
                    "Rank",        
                    RANKX(
                        ALLEXCEPT('Sprint Snapshots', 'Sprint Snapshots'[Column]),
                        CALCULATE( MAX ('Sprint Snapshots'[Sprint])),
                        ,
                        DESC,
                        Dense
                    )
                )
            , [Rank] <= 3)
        , [Sprint])

    var _AvgPointsDelivered = 
        DIVIDE(
            CALCULATE(
                sum('Sprint Snapshots'[Size])
                , 'Sprint Snapshots', 'Sprint Snapshots'[Sprint] in _Last3Sprints
                , 'Sprint Snapshots', 'Sprint Snapshots'[Column] = "Done"
            )
        , 3)

    return
        CALCULATE(_AvgPointsDelivered)+0

What am I doing wrong here?

[sjoerdvn]

I think your issues are mainly caused by using 'Sprint Snapshots' as an argument to the calculate function. 
try this:

AvgPointsDelivered = 
    var _Last3Sprints = TOPN(
	3,
        CALCULATETABLE(VALUES('Sprint Snapshots'[Sprint]),ALL()),
	'Sprint Snapshots'[Sprint],
	DESC)

    var _AvgPointsDelivered = 
        DIVIDE(
            CALCULATE(
                sum('Sprint Snapshots'[Size])
                ,  _Last3Sprints
                , 'Sprint Snapshots'[Column] = "Done"
            )
        , 3)

    return
        _AvgPointsDelivered +0

[Fowmy]

@SquidgyWidget 

Share more data in Excel or Csv and show the expected result for a few users. You can share the file including a dummy PBI using Google Drive or One Drive, past the link here.