Thank you very much for your question. It is our pleasure to answer it.

Abu Hurairah reports that the Messenger of Allah (peace be upon him) said: "If you come to the Prayer and we are in sajdah, then make sajdah with us but do not count it [as a rakah]. And whoever 'catches' the ruku', he catches the (rakah of) Prayer." (Abu Dawud) 

If one happens to join a Magrib prayer in congregation and catches only the last (or third) rakah of the prayer, one should follow the imam leading the prayer until he makes the tasleem to end his prayer, then stand up and complete the number of rakahs one has missed for that particular prayer.

Thus if one catches the last (or third) rak`ah of the Magrib prayer, that would be considered one's first rakah. After the declarations of the tasleems of the imam, one should:

- Stand up, pray one rak`ah and recite Surah Al-Fatihah and another Surah, (and this rak`ah would be counted as the second rak`ah of their Magrib prayer)
- Then sit down and offer the first tashahhud.
- Then stand up and offer another rak`ah reciting only the Surah Al-Fatihah in it; and this would be their third and last rakah of their Magrib prayer.

I hope this answers your question.