The sequence is a geometric one where each number is multiplied by 3 to get the next number.
- \(3 \times 3 = 9\)
- \(9 \times 3 = 27\)
- \(27 \times 3 = 81\)
- \(81 \times 3 = 243\)
To find the next number after 243:
- \(243 \times 3 = 729\)
So, the missing number in the sequence is 729.