Lal Kitab refers to 3rd house as 'the times of exit from this world and the gateway of exit'. In other words we come to know about the time of death and its causes. As illness is one of the routine ways of exit we have to judge the illness and diseases from this house. This is an inauspicious and malefic house that way.
Malefics in Dusthanas or inauspicious houses are normally considered to be a good placement. According to Lal Kitab are placements of sinners meaning Rahu and Ketu in 3rd house is horrible. It is said that even if they are not able to bring about the end itself they shall be competent to bring about death-like position. This may happen when 3rd house is occupied by Rahu or Ketu and houses 6 and/or 8 by malefics. In adversities this house gets strength from 12th house even though the planets in 12th house are enemies of the planets in the 3rd house. In a hypothetical case let us assume that Ketu occupies 3rd house and Mars is posited in 12th house. Mars is an enemy of Ketu but in emergencies Mars will always help Ketu despite his enmity. Ketu shall have no ill effects upon the native. Likewise Jupiter in the 3rd house and Mercury his enemy, in 12th house will not be able to cause any harm. On the other hand there will be all round happiness and prosperity. If the houses are occupied by two or more planets we have to assess the harm, in any, in respect of each planet and then draw our final conclusions. Rahu and Venus in 12th house normally being about the death of the partner - wife or husband - in 24/25th year of the native. If Saturn occupies the 3rd house simultaneously Rahu will not be able to play his mischief. Saturn will help Venus.