If a given set has 26 elements, how many of its subsets have at most three elements?
There are subsets that have at most three elements.

Step 1 ：We are given a set with 26 elements and we are asked to find the number of its subsets that have at most three elements.

Step 2 ：The subsets of a set with 26 elements that have at most three elements are the subsets with 0 elements (the empty set), 1 element, 2 elements, and 3 elements.

Step 3 ：We can calculate the number of these subsets using the combination formula, which is \(nCr = \frac{n!}{(n-r)! * r!}\), where n is the total number of elements, r is the number of elements in the subset, and '!' denotes factorial.

Step 4 ：Let's calculate the number of subsets with 0, 1, 2, and 3 elements separately and then add them up.

Step 5 ：The number of subsets with 0 elements is \(\binom{26}{0} = 1\).

Step 6 ：The number of subsets with 1 element is \(\binom{26}{1} = 26\).

Step 7 ：The number of subsets with 2 elements is \(\binom{26}{2} = 325\).

Step 8 ：The number of subsets with 3 elements is \(\binom{26}{3} = 2600\).

Step 9 ：Adding these up, the total number of subsets with at most three elements is \(1 + 26 + 325 + 2600 = 2952\).

Step 10 ：Final Answer: The number of subsets of a set with 26 elements that have at most three elements is \(\boxed{2952}\).