If I understand your question correctly, the problem is that you use each combobox as a a separate filter which removes all previous filters
If you want to use all of them I suggest you handle all combo box selections in a single method and inside that method you use all the selections from all combo boxes.
Consider the following pseudo code
method FilteringCombo_SelectedIndexChanged
filter as string = ""
if FilterCombo1.SelectedIndex >= 0 then
filter = "Amount like '%" + FilterCombo1.Text + "%'"
end if
if FilterCombo2.SelectedIndex >= 0 then
if filter <> "" then
filter = filter & " AND "
end if
filter = "Day like '%" + FilterCombo2.Text + "%'"
end if
if FilterCombo3.SelectedIndex >= 0 then
if filter <> "" then
filter = filter & " AND "
end if
filter = "Name like '%" + FilterCombo3.Text + "%'"
end if
bs.Filter = filter
end method