Solve Three-Digit Addition: 115 + 236 Using Vertical Format

$\begin{aligned} &115 \\ +& \\ &236 \\ &\underline{\phantom{776}} & \\ \end{aligned}$

To solve this problem, we'll use the following steps:

• Step 1: Align the numbers 115 and 236 vertically by their place values:

$\begin{aligned} &115 \\ +&236 \\ &\underline{\phantom{776}} \\ \end{aligned}$

Step 2: Start by adding the digits in the units (right-most) column:
$5 + 6 = 11$
Write 1 in the units place and carry over 1 to the tens column.

Step 3: Add the digits in the tens column with the carry:
$1 + 3 + 1 = 5$
Write 5 in the tens place.

Step 4: Add the digits in the hundreds column:
$1 + 2 = 3$
Write 3 in the hundreds place.

Putting it all together, we have:
$\begin{aligned} &115 \\ +&236 \\ &\underline{351} \\ \end{aligned}$

Therefore, the sum of 115 and 236 is $\textbf{351}$. The correct choice is option 3.