If you are talking about a proclamation talking about "we have magic so don't you dare attack", I can't help you. If you are talking about more generally, the three kingdoms has one warlord who gave pensions to mystics, various uses of shaman and diviners in the kingdoms. There are biographies of diviners, dream interpreters, a doctor with an interest in it and the mystics like Zuo Ci in the records of the three kingdoms.

Don't worry, there is nothing wrong with asking about magic in the past. People believed in magic, there are some fun tales about it and it is reasonable to ask how it influenced things. You don't need to distance yourself from it, my answer would be the same on this topic if you believed in magic or not.

The Inspector of Qing, at the start of the civil war, Jiao He often used shamans and prayed to spirits but he tended to avoid battle whereas the Qingzhou Turbans grip in Qing grew without much concern for the magic users alongside him. When the Han court was under military junta control, the leading general Li Jue relied on sorcery and divination, charms to protect him and believed their magic was why he rose so high. When the Wu general He Qi was conquering the southern people, there is a tale from Ge Hong who had quite a collection of Wu mystic stories, where their magic blunted the swords and arrows of He Qi's army so he had wooden cudgels made to get around that

As the three kingdoms were formed from the civil war, each court had diviners of various arts like physiognomy dream interpretation, wordplay, use of omens, reading the heavens and other means. The warlords didn't always listen to them and nor were they expected to get everything right, interpretation was not a perfect art. Some warlords would go into more magical figures than such divination and interpretation.

Cao Cao, controller of the Han who by this point was the leader of the largest power, would give pensions to a series of mystics and as Rafe De Crespigny notes, one wonders if the amount of Cao Cao's children is due to his attempts to copy the techniques of mystics for long life. His eldest son and founding Emperor Cao Pi would attempt a clampdown on witchcraft and another son Cao Zhi felt the need to argue clear Cao Cao was protecting the public rather than believing in the mystics and made some effort to dismiss their abilities. The elderly Wu Emperor Sun Quan made the "immortal" Wang Biao a General (though never seems to have held command, more a symbolic rank) and ennobled him, setting up a residence and providing gifts, Wang Biao acted as a diviner but fled soon after Sun Quan became paralysed. The Wei Emperor Cao Rui put in his palace a farmer's wife blessed by the heavens, said to be able to cure people (then executed her when he was fatally ill and her magic water didn't work.)

Did armies get put off by these? No, the Qingzhou Turbans still fought in Qing and Jiao He was the one noted to avoid battle, Li Jue would end up fighting some of his officers and his former allies in brutal wars, He Qi won the day. Wei, Shu-Han and Wu didn't avoid attacking each other when Wei or Wu were employing mystics.

Why didn't it stop armies? Not everyone believed in mystics as anything more than charlatans. Sun Ce, the elder brother of Sun Quan, was firmly opposed to such men, perhaps slightly awkwardly given the south's traditions with magic. Some mystics fled north, another was killed and said dead mystic Gan Ji's followers seem to claimed he haunted Sun Ce to death. Cao Rui's seriously unwell relative (via grandmother's side) and diabetic Bian Lan rejected attempts to cure him via the lady of the palace. Cao Zhi sought to play down the interest of his father as if rather embarrassed by the whole thing and felt the need to explain it as an act of protection so the family didn't look foolish.

The main theme for a three kingdom mystic was nourishing one's vital essence via controlled breathing, appropriate food, sleeping habits and the like, done correctly it would make them appear younger than they were and they lived for centuries (or they lied about their ages). These arts allowing with some having to knowledge to heal drew followers and tales of magic surrounded them. Mystics were credited with such deeds as walking on water, turning into sheep, haunting to death, control of the weather and nature, ability to move with speed, seeing the future, summoning food from distance and instant messaging among other things. Witchcraft and shamans could be used for spirits, for curses (a charge of witchcraft in the harem could be deadly), for prayer and there was always medical cures.

Such mystics were... of more use in court life if one was so inclined, figures to help seek long life, to help with one's health and that of your friends via long life practise or healing efforts, to help predict the future and read the heavens. Not so much to help in military affairs and one would have to wonder exactly how it would have gone down (other then likely very badly) if any ruler had dared used one as such as a miliatry strategy or avoided invading due the other court having a mystic.

Hope this helped and if you don't have follow up questions then have a good weekend

Sources:

Records of the Three Kingdoms by Chen Shou, annotations by Pei Songzhi, translation by Yang Zhengyuan

A Biographical Dictionary of Later Han to the Three Kingdoms 23–220 AD by Rafe De Crespigny

Generals of the South: The Foundation and Early History of the Three Kingdom State of Wu by Rafe De Crespigny

Imperial Warlord: A Biography of Cao Cao 155-220 AD by Rafe De Crespigny