whole bar sixths with additional notes

Hi. How do I add, in a single measure, a sixth (middle C and A) for 3 beats (in 3/4 time) and include 6 eighth notes that strike out individually while holding the C and A.
Likewise, I want to add, in a single measure, a 3rd (E and G) for 3 beats, while indicating a quarter note rest for the first two beats, followed by two eighth notes.

But when I select a single note in the sixth, or in a chord, and try to change the duration for just that note, it changes all the note values.

thanks