Given sequence: 3, 14, 39, 84, *, 258

The pattern follows a cubic relation:

• For 1: $1^3 + 1^2 + 1 = 3$
• For 2: $2^3 + 2^2 + 2 = 14$
• For 3: $3^3 + 3^2 + 3 = 39$
• For 4: $4^3 + 4^2 + 4 = 84$
• For 5: $5^3 + 5^2 + 5 = 155$
• For 6: $6^3 + 6^2 + 6 = 258$

Thus, the missing number is 155.