Q1. I've had my computer for more than five years.
Q2. She has spoken 3 languages since she was a child.
Q3. It's ok. I've only been waiting for a few minutes.
Q1: The correct answer is for. The sentence 'I've had my computer for more than five years' indicates that the computer has been in the student's possession for a certain period of time.
Q2: The correct answer is since. The sentence 'She has spoken 3 languages since she was a child' suggests that the student started speaking the languages in her childhood and continues to do so.
Q3: The correct answer is for. The sentence 'It's ok. I've only been waiting for a few minutes' indicates the duration of time the student has been waiting for.