I'm not sure what you've already done.
When you make a morph, you should always be sure you start at the base. Then you click on new morph to create the next. If you don't, you are adding to the previous morph.
In layout after adding the morph mixer; you should have three sliders, one for each of the morphs you made. You will see a "K" at the right of the sliders. Click this to set a keyframe. Make it a habit to set a Keyframe for each time you maka a change. That includes closing the mouth back to normal.
The morph mixer allows you to mix several morphs at the same time, this is why you want to do each morph separatley.
I hope this helps you out
