Spirited definitions...

Attention is when interest is captured.

Concentration is when Attention is narrowed.

Focus is when Concentration is intensified.

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Merriam-Webster dictionary:

Attention

• the act or power of carefully thinking about, listening to, or watching someone or something
• notice, interest, or awareness
• special care or treatment

Concentration

• the ability to give your attention or thought to a single object or activity
• the ability to concentrate
• the act of giving your attention to a single object or activity
• a large amount of something in one place

Focus

• a subject that is being discussed or studied 
• the subject on which people's attention is focused
• a main purpose or interest
• a point at which rays of light, heat, or sound meet or from which they move apart or appear to move apart
• the point at which an image is formed by a mirror, a lens, etc.