Explanation:

- Let the nine consecutive even numbers be: $x,x+2,x+4,\dots ,x+16$.

- Their average is given as 24. Therefore:

$(x+(x+2)+(x+4)+\dots +(x+16))/9=24$.

- Simplifying, the sum is $9x+72$ and:

$(9x+72)/9=24$ so,

$x+8=24$ and solving gives $x=16$.

- So, smallest three numbers are 16, 18, 20.

- The average of these three numbers:

$(16+18+20)/3=18$.

- Option 1: 21 - Incorrect.

- Option 2: 20 - Incorrect.

- Option 3: 18 - Correct.

- Option 4: 19 - Incorrect.

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