Is overthinking violate third precept?

No it doesn't! And it's certainly not about doing something too much. It's about having a sexual relationship with someone else's wife, husband etc. Otherwise getting addicted to music, cakes, chocolates etc. would break the 3rd precept too.

The precept is defined in the suttas as "sexual bodily action":

And how is one made pure in three ways by bodily action?... Abandoning sensual misconduct, he abstains from sensual misconduct. He does not get sexually involved with those who are protected by their mothers, their fathers, their brothers, their sisters, their relatives, or their Dhamma; those with husbands, those who entail punishments, or even those crowned with flowers by another man.

AN 10.176

Since the suttas (DN 31) also teach parents must arrange the marriage of their children and also teach the taint of woman is sexual unchastity, the precept of sexual misconduct also includes sex outside of marriage, i.e. sex without commitment, without compassion, motivated only by lust.

As for sensual pleasures in general, there are many suttas, such as MN 75 and MN 54, which describe the negative aspect of sensual pleasures however these suttas do not appear to use the term "misconduct":

So too, Māgandiya, formerly when I lived the home life, I enjoyed myself, provided and endowed with the five cords of sensual pleasure: with forms cognizable by the eye…with tangibles cognizable by the body that are wished for, desired, agreeable, and likeable, connected with sensual desire and provocative of lust. On a later occasion, having understood as they actually are the gratification, the danger, and the escape in the case of sensual pleasures, I abandoned craving for sensual pleasures, I removed fever for sensual pleasures, and I abide without thirst, with a mind inwardly at peace. I see other beings who are not free from lust for sensual pleasures being devoured by craving for sensual pleasures, burning with fever for sensual pleasures, mn.i.506 indulging in sensual pleasures, and I do not envy them nor do I delight therein. Why is that? Because there is, Māgandiya, a delight apart from sensual pleasures, apart from unwholesome states, which surpasses even divine bliss. Since I take delight in that, I do not envy what is inferior, nor do I delight therein.

2“Suppose, Māgandiya, there was a leper with sores and blisters on his limbs, being devoured by worms, scratching the scabs off the openings of his wounds with his nails, cauterising his body over a burning charcoal pit. Then his friends and companions, his kinsmen and relatives, would bring a physician to treat him. The physician would make medicine for him, and by means of that medicine the man would be cured of his leprosy and would become well and happy, independent, master of himself, able to go where he likes. Then he might see another leper with sores and blisters on his limbs, being devoured by worms, scratching the scabs off the openings of his wounds with his nails, cauterising his body over a burning charcoal pit. What do you think, Māgandiya? Would that man envy that leper for his burning charcoal pit or his use of medicine?”

“No, Master Gotama. Why is that? Because when there is sickness, there is need for medicine, and when there is no sickness there is no need for medicine.”

“So too, Māgandiya, formerly when I lived the home life…as in §12…Since I take delight in that, I do not envy what is inferior, nor do I delight therein.

MN 75

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Householder, there are these eight things in the Noble One’s Discipline that lead to the cutting off of affairs. What are the eight? With the support of the non-killing of living beings, the killing of living beings is to be abandoned. With the support of taking only what is given, the taking of what is not given is to be abandoned. With the support of truthful speech, false speech is to be abandoned. With the support of unmalicious speech, malicious speech is to be abandoned. With the support of no rapacity and greed, rapacity and greed are to be abandoned. With the support of no spite and scolding, spite and scolding are to be abandoned. With the support of no anger and irritation, anger and irritation are to be abandoned. With the support of non-arrogance, arrogance is to be abandoned. These are the eight things, stated in brief without being expounded in detail, that lead to the cutting off of affairs in the Noble One’s Discipline.

MN 54

In simple terms: Precepts = refraining from actions.

My understanding is that the 5 Precepts not only helps us from creating negative karma (Kamma = intention + actions) but also helps us in cultivating a foundation to release our attachment to the 5 Skandhas. (Actions being external factors of attachments.)

To your question:

[My question is how fair or correct to narrow third precept to sexual misconduct or adultery?] <--- I do not see any bits of "unfair" or "incorrectness" to limit sexual misconduct to the third precept. No matter in Theravada or Mahayana, they have given numerous examples and conditions on what is considered as Sexual Misconduct and inappropriate actions. And the range of stuff is very wide.

[Does over-thinking violate third precept because its a misconduct of mind due to uncontrollable and misuse mind?] <--- No. The 5 precepts corresponds to actions only. Over-thinking is addressed through training the mind.

A method of training one's mind is to observe how they arise from the 3 poisons: Greed, Hate, Ignorance, and then apply respective antidotes (Generosity, Loving-kindness, and Wisdom). This training is obviously the core teachings of the Buddha, i.e. the 4 noble truths and 8 folds paths.