Question 1 with 1 blanksomos tres hermanos: tengo dos hermanos mayores y yo soy (joven). question 2 with 1 blanksi entrenas cada día puedes ser la (buena) jugadora en tu equipo. question 3 with 1 blankel vaticano (vatican) tiene una población de sólo setecientos setenta personas y es el estado

These are the answers:

1. El más joven.

2. La mejor.

3. Más pequeño del mundo.

Explanation: Just to elaborate a little on the answer, it can be added that in this exercise they are asking you to fill the blank with a superlative adjective. Like in English, some adjectives in Spanish have a specific superlative form (bueno, mejor, el mejor; malo, peor, el peor), whereas others turn into a superlative form with the addition of the word "más" (more). In the first sentence, the speaker has two elder brothers, so he is the youngest. In the second sentence, the player might become the best player in her team if she trains hard. In the third sentence, Vatican City is described as the smallest state in the world.