Water from the tap is absolutely valid for baptism.  Perhaps you should speak with your confessor about scrupulosity.
It is also a very serious sin to repeat a valid Sacrament, as well as to administer a Sacrament conditionally without good reason.  

As to the original question, as long as water flowed onto the head at some point while the form was being pronounced, the baptism is valid.  The fact that the priest ran out of water as he said "..et Spiritus Sancti." would not invalidate the baptism.   The Ritual states three distinct pours for the rite, but this is not necessary for validity.

Correct.  There's no requirement for validity that there be a specific number of infusions or that the infusions must take place exactly when certain words are pronounced.  These kinds of things are prescribed by the ritual for various symbolic reasons and not in the interests of validity.  So long as the proper words are pronounced by a proper minister with the proper intention and water is poured on the head in such a way that the water flows across the skin, the Baptism was certainly valid.

I know one guy who questioned the Baptism of his kids because the priest said "Holy Spirit" instead of "Holy Ghost".  You can drive yourself nuts with this kind of thing.

Matter, form, and intent is all that matters.
Matter: Water looks very different depending upon where you are in the world.  I was told that in an extreme emergency spit could be used.

Form: "I baptize thee in the name of the Father, and of the Son, and of the Holy Ghost," while pouring water on the head.  The catechism does not say the water must flow, nor does it say the pouring must be done three times.  The catechism does say, in case of necessity, water may be poured anywhere on the body, not absolutely requiring the head.

Intent: you can never fully know the intent.

As a bishop recently told me, keep our Faith simple.  God is not complicated.  He knows our intent.  As st. Bonaventure said, "When we pray, the voice of the heart must be heard more than that proceeding from the mouth."

Be very careful here.  Spit and pouring on other parts of the body (other than the head) are considered DOUBTFUL for validity.  That's why they can only be used in an extreme emergency where nothing else can be done.  You can NEVER use something that's CLEARLY invalid.  But in extreme necessity it's possible to use doubtful matter.  While the catechism does not explicitly state that the water must flow, that requirement is taught by theologians.  Now, it's next to impossible for water to touch the skin and not move (i.e. flow) ... so maybe that's why the catechism doesn't mention that.  Baptism involves a WASHING with water, and washing by definition means that the water must move across the skin.

And, yes, you CAN know the intent.  If a person performs the rite as the Church prescribes, then the intent to DO what the Church DOES is there.

Extreme emergency is what I said.  God knows our intent.
It is VERY possible for water to touch the skin and not flow.  Washing can involve the water being moved across the skin, not necessarily water moving freely across the skin.  For instance, sponge bathing using damp washcloths.
 if intent were automatic based on matter and form, then the Church would not require matter, form, and intent to make the sacrament valid.

Yeah, well, it needs to be made clear that these emergency procedures are of DOUBTFUL validity.

No, it's hardly possible for water to touch the skin and not flow.  Unless it's on a perfectly flat surface, water moves.

You don't understand intention properly, since the way you explain it no one can every really know if any given Sacrament is valid.

Catholics are required to have moral certainty about the sacraments they receive. 
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If you receive a sacrament about which you are doubtful (not scrupulously doubtful, but about which you have reason to doubt), you sin.
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Intention is presumed when a Catholic minister uses the correct matter and form. Then validity is certain.

Exactly my point, that no one can ever be completely certain that a sacrament is valid.  One can only do their best and leave the rest to God.

False.  There's a lot of borderline-insane scrupulosity among Traditional Catholics.

Mith wrote "intention" not "input". If you read what's written rather than what you think, it would help you to "see it", don't you think?

I understand what the question means.  How can VALIDITY (outcome) be certain when the elements required for validity (input) are doubtful?  It's a valid question.  I disagree that intention cannot be known so I reject this question as founded on an illegitimate premise.  But this poster has a point that the validity cannot be certain if one cannot know intention with certainty.