Use (in a calculated column):
=CALCULATE(MIN([Column3]),FILTER(myTable,myTable[Column1]=EARLIER(myTable[Column1])),myTable[bColumn])
where myTable[bColumn] is the calculated column
=myTable[Column1]<>myTable[Column2]
If you want the formula in a measure instead of a calculated column, use:
=CALCULATE(MIN([Column3]),FILTER(ALL(myTable),COUNTROWS(FILTER(myTable,myTable[Column1]=EARLIER(myTable[Column1])))),myTable[bColumn])
HTH!