Ketu is the planet of spiritual liberation and detachment, and it has a unique influence on our lives. When placed in the third house for lagna Sagittarius, Ketu's effects become multi-dimensional as it aspects the 5th, 9th, and 12th houses.
The third house represents communication, siblings, intellectual pursuits, courage, and enthusiasm. The combination of Ketu in the third house of communication can make individuals introverted, detached from communication with others. They may become loners or prefer private conversations over socializing with new people. Those under this placement are likely to struggle with a lack of articulation and appropriate self-expression.
As Ketu also affects intellectual pursuits in the third house, individuals may tend to always pursue abstract ideas rather than practical ones. Although they have sharp minds that can grasp complex concepts more surrealistic in nature. They will still have a natural curiosity about all things spiritual.
Moreover, Ketu's aspect on the fifth house brings about a sense of detachment from parenthood or having children in their lifetime as the fifth one signifies creativity and children. Therefore these individuals may have fewer chances to experience good opportunities concerning children or creative projects.
The ninth house represents fortune, higher learning/education which provides great spiritual insights/philosophy to life including dharma (righteousness), philosophy & belief system while also representing long journeys/travels. With ketu's influence on this significant ninth house transits placed at such an important angle-like aspect will significantly affect such aspects associated with the ninth house.
While traditionally believed that blessings from Parvati and Lord Shiva get showered upon these natives when Ketu is placed in this manner for Lagna Sagittarius; it may not be true for everyone out there as it varies according to individual horoscopes but it can definitely help in predicting certain trends wherein they might get blessed by some sort of spiritual advancement during their journeys. But on the flip side of this, seeking guidance from astrologers in case these results are seen negative or unfavorable is recommended.
Lastly, Ketu's transit in the 12th house will mostly influence spiritual endeavors and dreams as it represents detachment from worldly pursuits and reaching a state of self-realization. This placement can signify that these people will become more inclined towards pursuing spirituality for enlightenment rather than leading an overly luxurious materialistic lifestyle. Some people are born with such placements and may stay away from too much indulging/pampering themselves while prioritizing detachment, ideals, philosophy and higher truth. Particularly for Sagittarius Lagna natives where both houses (Sagittarius & Pisces) signify spiritual awakening purposes & realization.
In conclusion, Ketu's placement in the third house with aspect on the fifth, ninth, and twelfth houses will significantly affect a person's communication skills leading to detached communication with others but better understanding of spiritual concepts. It may also affect their fortune-related travel opportunities but guide them towards higher learning & philosophical wisdom that stimulates their intellect. Lastly, it may lead individuals towards spiritual enlightenment/goals by detaching them consciously from material pleasures of life. Therefore for those who have this particular planetary position must focus on strengthening their faith upon spirituality and the divine which would help them attain inner-peace throughout their lives irrespective of ups/downs they might encounter.