The 3rd house in the ascendant (Lagna) chart represents short journeys, relations of the native with their siblings, number of siblings, health and longevity of the siblings, courage, writing, contracts, adventures, etc.
Jupiter in the 3rd house would naturally not be too comfortable as Jupiter is about stability but the 3rd house is about courage, valour, and adventures – all of which would be far too fast-paced and dynamic for a planet set in traditions and cultural settings like Jupiter.
For this reason, the 3rd house is referred to as the Marana Karaka Sthana (MKS) for Jupiter; the MKS is a house where a certain planet becomes powerless or loses much of its powers to influence the life of the person.
However, recall that there are two kinds of strengths in Vedic astrology – strength of the zodiac sign and the strength of the house. In the Marana Karaka Sthana, Jupiter will suffer somewhat due to the strength of the house being reduced, but if He is placed in a good zodiac sign such as Cancer, Pisces, or Sagittarius, He will give good results.
As such, if Jupiter is well-placed in the 3rd house, the person will share a good relation with his/her siblings; the health of the siblings would be decent and they will also have a decent lifespan unless Jupiter is afflicted. Furthermore, the person may undertake short journeys to attend ceremonies from time-to-time, or they may undertake short trips for religious purposes.
However, a person with Jupiter in their 3rd house will be too cautious or careful about money, bordering on being miserly.
Some considerations for Jupiter in the 3rd house are given below:
Jupiter in 3rd house in Cancer
This placement is applicable to Taurus ascendants; Jupiter is exalted in Cancer and so in the 3rd house, He will be able to provide excellent benefits to the native even though the 3rd house is considered to be the Marana Karaka Sthana for Jupiter. The person will be respected by their siblings and also earn decent wealth, presumably because of the 7th aspect of exalted Jupiter being cast upon the 9th house which controls fortune and destiny.
Jupiter in 3rd house in Sagittarius
This placement is applicable to Libra ascendants; Jupiter in Sagittarius is strong because Sagittarius is a fire sign and in the 3rd house, fire signs perform well due to the nature of the house being adventurous (which explains why Mars is the natural significator or Karaka of the 3rd house). Jupiter in the 3rd house in Sagittarius will very likely give the person at least one brother who will be successful and long-lived unless Jupiter is afflicted. The person will be active and also exert energy into their daily work as well as occupation; they might also take short trips to religious places.
Jupiter in 3rd house in Pisces
This placement is applicable to Capricorn ascendants. Pisces is an emotional zodiac sign; the person with a Jupiter in their 3rd house being Pisces will likely have only female siblings or more female siblings than male siblings. Such an individual will enjoy material comforts, but they will also be emotional towards their siblings and end up spending more on them.
Other Notes
Jupiter does not have any true enemies except Rahu; if Jupiter Rahu conjunction happens in the 3rd house then the life of the siblings of the person could be in danger. This could also happen if a weak Jupiter is aspected by a relatively strong Rahu. If Rahu happens to be the lord of the 8th house (meaning that the 8th house is Aquarius since Rahu co-rules Aquarius along with Saturn) and forms a conjunction with Jupiter, then the person’s life could potentially be disrupted or they could suffer mortal harm during their short journeys.
Jupiter Ketu conjunction in the 3rd house will make the person detached from his/her siblings; they will also remain largely indifferent to their matters of the house. Here, if Jupiter is the lord of the 12th house, it is possible that the individual might travel to foreign lands or move out of his city or town due to spiritual or religious reasons.
Jupiter with any benefic conjunctions will further improve the good effects of the chart; however, the sign placement also matters. Consider an example where Jupiter is placed in a friendly sign such as that of Mars, Moon, or Sun, with Mercury. So, the effects of Jupiter in the placement will be stronger than that of Mercury. However, if Jupiter Mercury conjunction happens in a sign which is not friendly for Jupiter, then Mercury’s effects will be stronger.