DAX measure return top 3 dates for month Year / Month pivot

Hi,
i have a measure which almost works;

 Top3day:=VAR top1 = TOPN(3, VALUES( 'Calendar'[Date]), [TotalUnits], DESC) 
VAR result =
IF( [TotalUnits],
 CONCATENATEX( top1, 'Calendar'[Date], ",  ", [TotalUnits], DESC)) 
 RETURN  
 IF( ISFILTERED('Calendar'[Month]), result , BLANK() )

Now this looks like it works but when it get's to the last Month in the table it goes mad and return all dates?

I have included a power query version to check results, this returns all dates above or equal to 3rd highest "units" value for each Year/ Month group.
When I change the measure to top 1 , then it works , not sure what's happening?

Richard.

Thanks, that does give a correct result, so lets see, if there a less than N values then all are returned ? is that it summed up.
I have not gone through your measure but will do. thanks of the help. Can't give a rep but would if could.

Richard.

so they are all tied for second and third place,