Hint: When the third note of the overtone sequence is still kind of difficult to realize, it can help to first play it with the correct fingering, to listen carefully and to then sing the respective note in the correct position. In male voices, the tone can certainly lay in the head voice but should still be easy to sing. The correct intonation is crucial, that's why you should compare it again with the correctly played tone.
Then play the overtone again with the feeling of the sung tone.