To simplify a three-term ratio, divide all terms by their highest common factor.<br><br>
&bull; The HCF of 3, 6, and 18 is 3.<br>
&bull; 3 ÷ 3 = 1<br>
&bull; 6 ÷ 3 = 2<br>
&bull; 18 ÷ 3 = 6<br>
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Final Answer: 1 : 2 : 6