SAP query

Hello ABAP Guru

I need a help here. trying to create SAP query joining three table VBAK VBAP KONV since KONV being cluster table I am not able to do that

thpough I am suceesfully establishe dthe link

VBAK-VBELN=VBAP-VBELN

VBAK-KNUMV=KONV-KNUMV & VBAP-POSNR=KONV-KPOSN

OSS msg says we have to write routine in sap query

but I never used that option

anyone has any clue on how to use cluster table in SAP query?

Thx

Praveen