You need to do few things:
1. Leave A3 validation list as it is now.
2. Create named ranges for each ISMC, ISJC, ISLC. Each named range includes respective sub-menu (ISMC 75x40 and alike items - only the column with description, not numberic values). Give each named range the name of ISMC (e.g. ISJC_Junior_Channel).
3. In B3 define Data validation as Custom: =INDIRECT("A3").
4. In C3 insert VLOOKUP that takes B3 value as lookup value and returns value of kg/m3 from sub-menu.

Thus, when you make selection in A3, B3 will return the list of items in corresponding named range only. Then, when you choose an item from the list, C3 will lookup that value in sub-menu and return corresponding kg/m3.

this is what you do..
in your data validation box in cell B3 change the value of the data validation to look like this

=IF(A3=A6,Sub_Menu_of_ISJC,IF(A3=A5,Sub_Menu_of_ISMC,IF(A3=A7,Sub_Menu_of_ISLC,"")))

then in cell C3 put this formula

=IFERROR(IF(AND(A3=A5),VLOOKUP(B3,B11:C30,2,FALSE),IF(AND(A3=A6),VLOOKUP(B3,B33:C37,2,0),IF(AND(A3=A7),VLOOKUP(B3,B40:C50,2,0)))),"")

what this does is take the information from cells A5 to A7.. compares it to the data validation in cell A3.. then changes the data in cell C3 to match.. and when you choose the data in the B3.. it will return the proper info in cell C3
hope this works.. :)

Waiting for valuable reply from Excel Expert, can any one find solution for the Post