Dear Marco,
this question is quite interesting. What Christian thinkers consider as being the soul is for us two distinct elements: the daena (the Conscience, a word which in later Avestan means religion, and has given the word "dēn" in Pahlavi, and "dīn" in modern Persian) and the urvan. The difference between the two is showed by many parts of the Gathas (ex. 31:20). While daena is dependent and created for each man, separately (31:11), the urvan contains the fravashi, God's own self distributed in the Creation. The fravashi obeys the laws of God only, and is pure, there is the same godly fravashi for every individual of the world.
To fully understand the story as told in many Zoroastrian books, the Avesta to begin with, one must acquaint one's self with Zoroastrian symbolism, a longer and very interesting subject. We have yet to consider the story of the death soul's journey as a most symbolic one, while being all true.
At every human death, stands the urvan with the fravashi by the head (a symbol of man's own decisions). If the good deeds, words and thoughts, did indeed exceed the evil ones, the fravashi sings the Ushtavaiti gatha, that is, the most joyous part of Revelation. If it is the obverse, it sings the Yasna 46, the saddest (at least, in appearance) of the Gathas. Then the urvan goes to the Bridge of Passage, and meets there either a beautiful maiden, or an ugly one. It is the daena in fact; the actions of man, which are that maiden. This is said about it:
"And his own deeds of a virtuous kind come to meet him in the form of a maiden, (126) who is handsomer and better than every maiden in the world. 127. 'And the righteous soul speaks (128) thus: "Who mayst thou be, that a maiden who is handsomer and better than thee was never seen by me in the worldly existence?"
129. 'In reply that maiden form responds (130) thus: "I am no maiden, but I am thy virtuous deeds, thou youth who art well-thinking, well-speaking, well-doing, and of good religion! 131. For when thou sawest in the world him who performed demon-worship, then thou hast sat down, and thy performance was the worship of the sacred beings. 132. And when it was seen by thee that there was any one who caused oppression and plunder, and distressed or scorned a good person, and acquired wealth by crime, then thou keptest back from the creatures their own risk of oppression and plunder; (133) the good person was also thought of by thee, and lodging and entertainment provided; and alms were given by thee to him (134) who came forth from near and him, too, who was from afar; and wealth which was due to honesty was acquired by thee. 135. And when thou sawest him who practiced false justice and taking of bribes, and false evidence was given by him, then thou hast sat down, and the recitation of truth and virtue was uttered by thee. 136. I am this of thine, the good thoughts, the good words, and the good deeds which were thought and spoken and done by thee. 137. For when I have become commendable, I am then made altogether more commendable by thee; (138) when I have become precious, I am then made altogether still more precious by thee; (139) and when I have become glorious, I am then made altogether still more glorious by thee."" (Menog-i Xrad II:125-139).
Thus there is wedding between the fravashi, the godly soul, and the daena, the manly soul; if the maiden is beautiful (that is, if the past life was good in essence and value and measure). Then follows a description of that evil soul of man's, which meets an ugly maiden, and there is no wedding. If there is a wedding, the wed is one, two no more, and join God in the Glorious House of song. If there is no wedding, the urvan goes to God, but the daena goes to the infamous House of the Lie, until it is purified.
That was an explanation of the role of the "soul". Now, as to: why three days; this may be explained by the importance of the number "three" in Zoroastrianism. Does it really stay near the head during three days or is it only symbolism? Those questions cannot found a prepared answer and must be answered by your own daena.
It is not in a dead body, to answer you dear Marco, but next to the "head". Once again, I think the head is mostly symbolic, but anyone can disagree with me.
The rituals are the chanting of prayers, there are prayers for the dead, and also three gah prayers dedicated to the soul, made by a priest (or many priests). Those prayers will not change the fate of the soul, but they connect the world to the soul, and are bringers of many advantages. They also show that people do their duty to the departed.
That is the ritual for the soul; there are many other rituals, destined to the body, which are to long to describe here. It must be known that the body should absolutely not be touched after three hours after the death, a time at which we are actually sure the body is completely nasu and inhabited by Ahriman. (Nasu means "dejection, very dirty thing").
Have I well answered those questions of yours?