What is Trutiyā vibhakti

तृतीया विभक्ति or Instrumental Case of the noun-form represents the instrumental form in a sentence. The answer that comes when you ask a question "by/with what" or "with whom" is the instrument in the sentence.
As the name suggests the instrument is one with what subject of the sentence is performing (verb) the action.
e.g. He goes home by cycle, He cuts the wood with an axe

The instrument is independent of the number and gender of the subject or object.

Trutiyā Vibhakti sentences in Sanskrit

1. सः शरेण हन्ति (He kills with an arrow)
In this sentence, सः (he) is the subject and he is killing with an arrow. शरेण represents instrumental form of शर.

2. राम सीतया सह गच्छति (Ram goes with Sīta)
In the above sentence, राम is the subject and सीतया represents the instrumental form of सीता.

In sentence 1 above, if we ask the question, 'with what?' the answer that we get is 'with arrow'.
In sentence 2 above, if we ask the question, 'with whom?' the answer that we get is 'with Sīta'.
So, in general, answer to the question 'by/with what' or 'with whom' is the instrument of the sentence and is always in instrumental case.

Examples of Trutiyā Vibhakti in Sanskrit

Trutiyā Vibhakti of Ram

विभक्ति	एकवचन	द्विवचन	बहुवचन
प्रथमा	रामेण	रामाभ्याम्	रामैः

Trutiyā Vibhakti of Lataa

विभक्ति	एकवचन	द्विवचन	बहुवचन
प्रथमा	लतया	लताभ्याम्	लताभिः

Trutiyā Vibhakti of Fal (Phal)

विभक्ति	एकवचन	द्विवचन	बहुवचन
प्रथमा	फलेन	फलाभ्याम्	फलैः