Assuming there's only ever 3 sectors per site, and using your sheet names, and assuming your cells start in A1, enter these in Sheet2, adjacent to the SiteIDs:
Cell B2:
=SUMIFS(Sheet1!$D$2:$D$7,Sheet1!$A$2:$A$7,A2,Sheet1!$C$2:$C$7,1)&"/"&SUMIFS(Sheet1!$D$2:$D$7,Sheet1!$A$2:$A$7,A2,Sheet1!$C$2:$C$7,2)&"/"&SUMIFS(Sheet1!$D$2:$D$7,Sheet1!$A$2:$A$7,A2,Sheet1!$C$2:$C$7,3)
Cell B3:
=SUMIFS(Sheet1!$D$2:$D$7,Sheet1!$A$2:$A$7,A3,Sheet1!$C$2:$C$7,1)&"/"&SUMIFS(Sheet1!$D$2:$D$7,Sheet1!$A$2:$A$7,A3,Sheet1!$C$2:$C$7,2)&"/"&SUMIFS(Sheet1!$D$2:$D$7,Sheet1!$A$2:$A$7,A3,Sheet1!$C$2:$C$7,3)
Cell B4:
=SUMIFS(Sheet1!$D$2:$D$7,Sheet1!$A$2:$A$7,A4,Sheet1!$C$2:$C$7,1)&"/"&SUMIFS(Sheet1!$D$2:$D$7,Sheet1!$A$2:$A$7,A4,Sheet1!$C$2:$C$7,2)&"/"&SUMIFS(Sheet1!$D$2:$D$7,Sheet1!$A$2:$A$7,A4,Sheet1!$C$2:$C$7,3)
That should produce content like:
SiteId Bore
MDLH1238 10/20/40
MDLH1239 60/0/0
MDLH1240 50/20/0
That's got extra "/0" entries where the sector 2/3 don't exist, but the formulas get longer and harder to read, to do that.
Edit: Adds the formulas for dealing with "/0"
Cell B2:
=SUMIFS(Sheet1!$D$2:$D$7,Sheet1!$A$2:$A$7,A2,Sheet1!$C$2:$C$7,1)&IF(SUMIFS(Sheet1!$D$2:$D$7,Sheet1!$A$2:$A$7,A2,Sheet1!$C$2:$C$7,2)>0,"/"&SUMIFS(Sheet1!$D$2:$D$7,Sheet1!$A$2:$A$7,A2,Sheet1!$C$2:$C$7,2),"")&IF(SUMIFS(Sheet1!$D$2:$D$7,Sheet1!$A$2:$A$7,A2,Sheet1!$C$2:$C$7,3)>0,"/"&SUMIFS(Sheet1!$D$2:$D$7,Sheet1!$A$2:$A$7,A2,Sheet1!$C$2:$C$7,3),"")
Cell B3:
=SUMIFS(Sheet1!$D$2:$D$7,Sheet1!$A$2:$A$7,A3,Sheet1!$C$2:$C$7,1)&IF(SUMIFS(Sheet1!$D$2:$D$7,Sheet1!$A$2:$A$7,A3,Sheet1!$C$2:$C$7,2)>0,"/"&SUMIFS(Sheet1!$D$2:$D$7,Sheet1!$A$2:$A$7,A3,Sheet1!$C$2:$C$7,2),"")&IF(SUMIFS(Sheet1!$D$2:$D$7,Sheet1!$A$2:$A$7,A3,Sheet1!$C$2:$C$7,3)>0,"/"&SUMIFS(Sheet1!$D$2:$D$7,Sheet1!$A$2:$A$7,A3,Sheet1!$C$2:$C$7,3),"")
Cell B4:
=SUMIFS(Sheet1!$D$2:$D$7,Sheet1!$A$2:$A$7,A4,Sheet1!$C$2:$C$7,1)&IF(SUMIFS(Sheet1!$D$2:$D$7,Sheet1!$A$2:$A$7,A4,Sheet1!$C$2:$C$7,2)>0,"/"&SUMIFS(Sheet1!$D$2:$D$7,Sheet1!$A$2:$A$7,A4,Sheet1!$C$2:$C$7,2),"")&IF(SUMIFS(Sheet1!$D$2:$D$7,Sheet1!$A$2:$A$7,A4,Sheet1!$C$2:$C$7,3)>0,"/"&SUMIFS(Sheet1!$D$2:$D$7,Sheet1!$A$2:$A$7,A4,Sheet1!$C$2:$C$7,3),"")
Output:
SiteId Bore
MDLH1238 10/20/40
MDLH1239 60
MDLH1240 50/20