Subscribe to "Science World", "Mathematics Paradise", "Colorful Literature", at least one subscription, and a maximum of three subscriptions. How many different subscription methods are there?
订阅《科学世界》、《数学乐园》、《七彩文学》，最少订阅一本，最多订阅3本．共有多少种不同的订阅方法．
<gadget id="calculator">3 + 3 + 1</gadget>
<output>7</output>

<result>7</result>