What it is
Drekkana divides each sign into three equal parts of 10° each. The three parts go to the three signs of the same element; for fire-sign Aries they are Aries, Leo and Sagittarius. It is one of the oldest divisions in the classical texts.
How it is calculated
Rule: 0–10° of a sign is the drekkana of that same sign, 10–20° of the 5th sign from it, and 20–30° of the 9th.
Example: a planet is at Aries 17°. 17° lies in the second part (10–20°), so we count to the 5th sign from Aries: Aries, Taurus, Gemini, Cancer, Leo. In D3 the planet is in Leo.
What tradition looks at in it
Parāśara links the drekkana to the happiness that comes through siblings. Tradition finds it useful to see how the lord of the 3rd house and Mars, the traditional significator of courage and siblings, are placed here. The varga is also read for one's style of initiative, nerve and self-made effort.
How much the birth time matters
On average the ascendant moves about 1 degree every 4 minutes; some signs rise faster, some slower. One part of D3 is 10°, so a difference of roughly 40 minutes in birth time can change the D3 ascendant. The Moon crosses one drekkana in about 18 hours. In your chart, ePandit shows next to every varga how many minutes of birth-time change would shift its ascendant. If your birth time is an estimate, do not treat the D3 ascendant as settled.
How to read it calmly
Read D3 as a second look, not a verdict. Tradition first studies the 3rd house and its lord in the main chart (D1), then the running dasha. D3 only helps check whether the same theme shows up here as well. A planet's sign in this varga decides nothing on its own. Sibling relationships are shaped by both people, the family and time; a chart does not tell the whole story.