Instead off praying 4 rakat farz namaz, (1) I make salam after third rakat or (2) I make salam after second rakat. How to make up for remaining rakat (s). Also consider the fact if I am praying alone or leading a jamat. Is Sajad Sahw is required.
If you missed a rakat in prayer you should repeat the salat as you missed an obligatory(fard) action.
It is reported in Sahih Bukhari Volume 001, Book 012, Hadith Number 724 as
Narated By Abu Huraira : Allah's Apostle entered the mosque and a person followed him. The man prayed and went to the Prophet and greeted him. The Prophet returned the greeting and said to him, "Go back and pray, for you have not prayed." The man went back prayed in the same way as before, returned and greeted the Prophet who said, "Go back and pray, for you have not prayed." This happened thrice. The man said, "By Him Who sent you with the Truth, I cannot offer the prayer in a better way than this. Please, teach me how to pray." The Prophet said, "When you stand for Prayer say Takbir and then recite from the Holy Qur'an (of what you know by heart) and then bow till you feel at ease. Then raise your head and stand up straight, then prostrate till you feel at ease during your prostration, then sit with calmness till you feel at ease (do not hurry) and do the same in all your prayers."
Making incorrectly the sujood and rukooh led to pray the salat again
Making salam after second or third rakats in a four rakaat prayer, both are same, you had missed some compulsory(fard) things in salat. So, you should pray again that four rakat.
If you are leading the jamaat means you should then re-organise the members(if available) and pray the salat again.
Sajda-sahw is required in a salat, when one forgets to perform wajib things like sitting in atthayaath after second rakaat is wajib. If you missed that means you need to perform Sajda-sahw. Here, you don't need to do that.
Sahih Bukhari Volume 001, Book 008, Hadith Number 394:
Narated By 'Abdullah : The Prophet prayed (and the subnarrator Ibrahim said, "I do not know whether he prayed more or less than usual"), and when he had finished the prayers he was asked, "O Allah's Apostle! Has there been any change in the prayers?" He said, "What is it?' The people said, "You have prayed so much and so much." So the Prophet bent his legs, faced the Qibla and performed two prostrations (of Sahu) and finished his prayers with Tasiim (by turning his face to right and left saying: 'As-Salamu'Alaikum-Warahmat-ullah'). When he turned his face to us he said, "If there had been anything changed in the prayer, surely I would have informed you but I am a human being like you and liable to forget like you. So if I forget remind me and if anyone of you is doubtful about his prayer, he should follow what he thinks to be correct and complete his prayer accordingly and finish it and do two prostrations (of Sahu)."
Sahih Bukhari Volume 001, Book 008, Hadith Number 398:
Narated By 'Abdullah : "Once the Prophet offered five Rakat in Zuhr prayer. He was asked, "Is there an increase in the prayer?" The Prophet said, "And what is it?" They said, "You have prayed five Rakat.' So he bent his legs and performed two prostrations (of Sahu).