Is the object you are trying to filter a subform?
If so then either in the onchange event for each combo or if you want a button, in the button on click event put code similar to this
YourSubFormName.Form.Filter = "[SomeField] = " & combo1 & " AND [SomeOtherField] = " & combo2
If the values in your combo are text then substitute
" & combo2 with '" & combo2 & "'"