As simple as your question sounds to you, I’m still not quite sure I get it. Sorry, I really am trying to help.
Are you new to playing music in general?
Half-holing comes into play when you’re trying to play a note that is not a natural note of the whistle. On a D whistle, the third note is F#. If you want to play a tune thats written in C, then you’ll have to half-hole that F# to get an F natural.
I think that this is becoming overly complicated though.
Play Mary Had a Little Lamb on a D whistle on the bottom three holes. You just played that in the key of D.
If you want to play it in the key of C, get a C whistle. If you want to play it in F, get an F whistle. Trying to play a whistle in the wrong key for a tune can be an exercise in frustration. That’s why they make different keyed whistles.
Now to go a step further, whistles can play in two different keys fairly easily with no half-holing. A D whistle can play in both D and G. There’s only one note that’s different (C natural - OXX OOO).
The thing to keep in mind regarding your question is that E is only one note. You have to consider the key of the tune. In general, the key of the tune is based on a major scale. So, if the tune is made up of notes in the D major scale, then that tune is in the key of D. A D whistle plays these notes automatically, therefore that’s the whistle you’d use for a tune in the key of D.
Maybe that’s where you’re getting tripped up. Don’t look at the individual notes, look at the key of the tune. The key is indicated at the beginning of the sheet music near the treble clef, and is designated by sharps and flats. If you see no sharps or flats, that’s the key of C. If there are two sharps, that’s the key of D.
Do a Google search on music theory for a general introduction to this stuff.
Sorry if you’re beyond all this, but it’s hard to tell from your question.
Hope that helps.
Jason