Valmiki Ramayana Ayodhya Kanda Sarga 54 Shloka 35
Original Shlokaसीतातृतीयः काकुत्स्थः परिश्रान्तः सुखोचितः। भरद्वाजाश्रमे रम्ये तां रात्रिमवसत्सुखम्।।2.54.35।।
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सुखोचितः used to comforts, परिश्रान्तः exhausted, सीतांतृतीयः Sita counted as the third, काकुत्स्थ: Rama, रम्ये in the lovely, भरद्वाजाश्रमे hermitage of Bharadwaja, तां रात्रिम् that night, सुखम् happily, अवसत् stayed.
The exhausted scion of the Kakutsthas, used to comforts, stayed in the lovely hermitage of Bharadwaja for the night with Sita, as the third member (of the group).