While Greg was camping with his family for a week, it rained for 3 days. When he looked at the weather records, he saw that the amount of rain was 3 mm, 6 mm, and 5 mm on the three days. During the same week, it rained 26 mm at his house. How much less rain did Greg experience while camping?
Please reason step by step, and put your final answer within \boxed{}.
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