Round 1:

Suns diameter is 1.392 million km. Thats 1392km x 106 which is 1,392km x 109

A average lion is 2m long. So 2m x 1012 would be all of them in a straight line. 2km x 109

That means that one trillion lions are longer than the diameter of the sun. But the sun is a ball, so its much bigger. Its diameter goes into every direction, not just in one.

Round 2: Assuming the lions are on Earth, sun wins by sitting out the fight. If the fight takes place in the middle of a city, the city vaporizes and the lions do too. Godstomp for the sun

Round 3: 1,989 x 1030 kg is the suns mass.

A lion weighs approx 250kg, so a trillion lions weighs 2,5 x 1014. the sun weighs far more than a trillion lions.

So its a 3/3 for the sun, correct me if i am wrong.