I think a solution of your problem. Your problem is actually parallel text change event on a same table.
You can prevent it by using got focus or click event of each combo-box, just use a variable like isCombobox2Working when user clicks on combox2 assign isCombobox2Working = true and then in textchange event check if the isCombobox2Working = true then work. When user clicks on Combobox3 set isCombobox2Working = false and in TextChanged event for combox3 check to work only isCombobox2Working = false.