***** NOT A TEACHER *****
Hello, Tufguy:
May I congratulate you on your excellent use of "whom"?
I think that your third example needs "speak to" or "speak with."
*****
It helps me to rearrange questions in order to better analyze them:
You did take it from ____?
It does belong to ____?
You do want to speak with / to ____?
As you can see, the word in the blank is the object of the preposition. Therefore, as you know, you need the objective form "whom."
James