The LCM of 3 and 7 is 21. There are several methods to find the LCM of two numbers, including prime factorization, listing multiples, and division method. One way to find the LCM of 3 and 7 is to use prime factorization. The prime factorization of 3 is 3, and the prime factorization of 7 is 7. To find the LCM, we multiply the prime factors raised to their respective highest power, which gives us 3^1 x 7^1 = 21. Therefore, the LCM of 3 and 7 is 21.