The numbers 1, 2, and 3 are written on the three cards, how many different natural numbers can be formed.
三张卡片上分别写着1、2、3三个数字，可组成多少个不同的自然数．
<gadget id="calculator">3 + 6 + 6</gadget>
<output>15</output>

<result>15</result>