Would help if you had posted the sql so we don't have to guess at what you've named things. I will guess that the field to be filtered is "Pkgs". Maybe for combo1 row source
Code:
SELECT * FROM qryGroups_Packages
then for combo2
Code:
SELECT * FROM qryGroups_Packages WHERE qryGroups_Packages.Pkgs <> Forms![Vehicle Entry Form].cboGroups_Packages_1
then for combo3
Code:
SELECT * FROM qryGroups_Packages WHERE qryGroups_Packages.Pkgs <> Forms![Vehicle Entry Form].cboGroups_Packages_1 AND qryGroups_Packages.Pkgs <> Forms![Vehicle Entry Form].cboGroups_Packages_2
and so on if there's more.
Perhaps you can see why I don't use such long names for controls. Using my naming convention it would have been frmVehEntry.cmbGrpPkg1 No brackets required; no underscore. Except for the spaces you have, yours is OK - just more cumbersome. If it were much worse, I might construct the row source in vba and assign it to the control. You're going to need code anyway, to requery the child combo(s) every time you select a value from a parent(s).
I also have no idea what the bound column is in the combos or even if what I wrote will match that column since * will return all fields. Hopefully you can figure out how to use your own SELECT part and incorporate the WHERE part of the examples.