Do you think Macbeth would become a great warrior instead of a tyrant if he did not meet the three witches and did not follow his wife's...

Of course, you know that what you are asking is pure
subjective supposition.  There is no "right" answer to a question such as this, since
the play, as it is written is all that we have.  It should be made clear that
Shakespeare set out to write a Tragedy here, and by the classical definition, the hero
of the tragedy must fall from a great height due to his own flawed perception of the
world -- his tragic flaw.

For Macbeth, this flaw is his
ambition, and it is in place in his nature whether he meets three witches and has a
pushy, power-hungry wife or not.  Shakespeare makes sure that the speech in which
Macbeth actually commits to his course of action, his soliloquy in Act I, scene vii, has
him alone onstage.  He considers all the very good reasons that it is a bad idea to kill
Duncan and then he decides to do it anyway.  His
reason:

...I
have no spur

To prick the sides of my intent, by
only

Vaulting
ambition...

and it is his
ambition that drives him throughout the play.  Shakespeare has also show how Macbeth
both moves away from his wife's influence and even decides to kill Macduff (though
Macduff eludes this) even though the witches' apparitions seem to suggest that Macduff
is not a threat.  Both of these actions show how he picks up more and more steam for his
own ambitious, murderous behaviour regardless of those around
him.

So, no, I do not think that the character Macbeth
would act in any other way.  If he did, we have a mildly pleasant story, but not the
great Tragedy that Macbeth is.  It is his fall, his all-too human
hubris, that the audience is meant to witness and pity.  Without this course of action,
there is no Tragedy of Macbeth.

As for
his being a great warrior.  Shakespeare goes to some trouble, at the opening of the
play, to have Duncan reward his actions in the most recent battle.  He praises Macbeth's
skills as a warrior:

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O worthiest
cousin!

....Would thou hadst less
deserved,

That the proportion both of thanks and
payment

Might have been mine!  Only I have left to
say

More is due than more than all can
pay.

It is important also to
the Tragic form of the play that Macbeth begin in this very noble and rather exalted
position.  This gives him the opportunity, through his own behaviour, to fall from a
great place.  He, himself, is not a king at the beginning of the play, but like Othello,
is a great warrior, praised and decorated for his skill and
bravery.

I think that a careful reading of the play and an
understanding of the classical requirements of a tragic hero will show that Macbeth is
already a great warrior and that he must act in the way that he does for the play to
fulfill itself as a Tragedy.