TABLE X
-------------------------------------------
COL SG COL VOL COL DB
-------- -------- --------
SG1 V1 T1
SG1 V2 T1
SG1 V3 T1
SG2 V4 T1
SG3 V5 T1
SG3 V6 T1


TABLE Y
-------------------------------------------
COL VOL COL SIZE1 COL DB
-------- -------- --------
V1 30 T1
V2 20 T1
V3 60 T1
V4 10 T1
V5 80 T1
V6 40 T1


TABLE Z
----------------------------------------------------------------
COL SIZE2 COL OBJ COl DB COL VOL
-------- -------- -------- --------
23 TS1 T1 V1
300 TS2 T1 V1
4 TS3 T1 V1
5 IX6 T1 V2
14 IX7 T1 V4
90 TS9 T1 V5


Need a query which results in
Z.OBJ values TS2 and TS9 .

TS2 and TS9 of Z.OBJ have a size Z.SIZE2 > size of Y.SIZE1 of any X.VOL
belonging to X.SG to which Z.OBJ belongs .

Remarks :
Z.VOL V1 belongs to X.SG1 which also has X.VOL V1, V2 and V3 so V1, V2 and V3 are good candidates in this case .
X.SG2 candidate is only V4
X.SG3 candidates are V5 and V6 .

Z.SIZE2 has the same format as Y.SIZE1 .

I don't understand what you want to do.

A 3-way join is quite simple: First you join only two tables. The result of that join is a (temporary) table. Since you have a table, you can join this with another table. Let's have a look at this:
To clarify this further, you could write it as:
The temporary table "join_tab" is the result of the join operation of "t1" and "t2". The second join step combines table "join_tab" with "t3".

Essentially, you have to figure out which row(s) in X belongs to which row(s) in Y - the condition describing this will be the join predicate for the join of X and Y. The same has to be done for X and Z and Y and Z. It may happen that you do not have to apply a join predicate on one of those three combinations.