The 3rd house is a rather typical house, it can become rather malefic with severe repercussions on the entire personality if it chooses to. It is a Upachaya house and is often a shadowy house.
The 3rd house in some Jyotish texts is considered as a house of turmoil/dushamsthan along with the 6th, 8th and 12th houses. It can definitely cause turmoil in the chart! I have observed that in the current age that we live in, this 3rd house can be quite useful (unless it is heavily damaged by malefics). We live in the age of Mercury, dominated by communication on a global scale, it is a superficial world, where we often act, need to act, fast and decisively. If this house is moderately influenced by malefics it can actually be an asset, as it gives a rough sort of courage and boldness, which is perhaps required today.
The third house is naturally ruled by Mercury with Gemini being the third sign of the zodiac. And where Mercury is, you can expect typical problems, mental thought patterns, ability to concentrate, willingness to act, stability of the mind, the way you think, desire to learn/study. The chest and the respiratory system. And because it rules the hands it is the first step to action! Hands are the most important tools you have, you touch, manipulate, use, push, pull, love with your hands. Your hands are how you act. Mercury, the impressionable adolescent of astrology here chooses to think and act.The basic birth chart will give the manifested reality and the 9th divisional chart/Navamsha gives the potential, evaluate both to get a true picture. You can build the picture using the keywords of the planet and the house together.
First the worst case scenario, most often seen in criminals. Now if this house is influenced by malefics alone, it can cause immeasurable harm. The Sun, Saturn, Rahu, Ketu or Mars if let loose here, without any support from the benefics Jupiter or Venus can wreck Mercury. Such a person can suffer physical damage often from miscalculation of risks while in action, e.g. while driving, in short journeys, while playing games, etc. Damage, delay, frustration in education, obstacles in work, emotional damage often projected as cruel and unstable behaviour. Physical deformity like arms amputated from birth. If the influencing malefics are of a similar nature then there is a tendency for self harm or early death either by suicide, accident or even as a punishment! If the influencing malefics are of dissimilar natures then the personality can be violent, crooked, criminal and heartless.
One of the worst placements in the chart would be a Ketu in the 3rd. This single planet here can cause havoc, especially if he is aspected by malefics and not by benefics. Even in transit such a lone Ketu can cause severe problems here. Mental restlessness, frustration, exiles, detentions, imprisonment, diseases of the chest and lungs etc. A Ketu in the 3rd would mean a Rahu in the 9th, which will impact the ‘foundations’ of your life. The help from the ancestors, peers and patrons will not be forthcoming. Also the deep philosophy that one lives his life by will be tainted by Rahu’s typical over-reaching, fake, acquisitive, greedy behaviour.
Saturn here will create a hesitant, overly cautious, cowardly personality who is scared to take initiatives in his life. He will also impact the intelligence, creativity and connection to the divine (aspect on 5th house), constrain the 9th house and give a negative impact on the entire personality and his environment by his aspect on the 1st house. Saturn+Ketu gives a violent criminal personality.
A Mars alone in the 3rd will create outbursts of uncontrolled force, death in accidents, injuries by sharp weapons, bloodshed, crimes of passion and temper, action without regard to consequences. A Mars aspected by benefics can create revolutionaries, brave, fiery, analytical military leader with a great sense of strategy and personal integrity. But Mars with other malefics can cause problems. Mars+Rahu will give brain/mental related problems at some point of life. A Mars+Rahu or Mars+Saturn can typically deny marriage or cause infertility. Especially if such a Mars is in a fiery sign then his effects will be more intense.
A Sun in the 3rd if poised to give bad results can give criminals who are ultimately executed by the government. Sun is also creativity, so such persons may be genius, artists, scientists, etc whose lives often take a tragic fatal turn. e.g. Ketu+Sun typically can give death by hanging, in fire or by poisons.
However you will rarely see such heavily afflicted charts in your regular life, these are the anti-social elements, oddly eccentric individuals whose career is generally cut short by death or isolation. A heavily afflicted 3rd house does not allow the native to roam at large for long. (A rather morbid study would be to analyse the charts of people who either committed suicide or were executed.)
If malefics are placed singly with/aspected by benefics in the 3rd house they can give good results. There would of course be conflicts in life, some delays and hindrances. This house belongs to the impressionable Mercury so he allows all the resident planets their expression initially in a juvenile manner. Thus its results improve with age as the planets and the houses mature. The best part is that here a good influence from Venus/Jupiter can counter-balance the malefic influences if any.
However in my opinion, the best option for the 3rd house is, it remains vacant and/or aspected by benefics.
(There was a question on the 3rd house being a dushamsthan and a upachhaya house etc, hope this post was of some help)