Meaning
Supreme God Krishna spoke
People like {King} Janaka attained the highest spiritual goal through detached action. You should also act {and fight the battle} for the public good, {even if not for personal gain or loss, which seems to be your primary concern at the moment}. (3:20)
Sanskrit
श्रीभगवान् उवाच
कर्मणैव हि संसिद्धिमास्थिता जनकादयः ।
लोकसङ्ग्रहमेवापि सम्पश्यन्कर्तुमर्हसि॥
Word-by-word
कर्मणा एव हि सम्-सिद्धिम्-आस्थिता जनक-आदयः।
लोक-सङ्ग्रहम् एव अपि सम्-पश्यन् कर्तुम् अर्हसि।।
Commentary
Arjuna has been thinking of the war in terms of his relationships and the consequences on his clan and family, to which he seems emotionally too attached. He does not seem to be equally considering the more significant public dimension of the war at hand.
It's worth noting that after emphasizing the importance of being "selfless," God now urges Arjuna and others to act for "Loka- Sangraha," which broadly means working for the welfare of the people. Such a call to philanthropy is rarely found elsewhere.