To calculate the total mass of the three particles, we perform the following addition:

$ \begin{aligned} & m_1 + m_2 + m_3 = 435.42 \, \text{g} + 226.3 \, \text{g} + 0.125 \, \text{g} \end{aligned} $

When adding numbers, the result should be reported with the same number of decimal places as the measurement with the fewest decimal places. Here, the second measurement (226.3 g) has only one decimal place, which is the fewest among the given values.

Thus, according to the rules for significant figures in addition, the sum should be rounded to one decimal place:

$ \begin{aligned} & \text{Total mass} = 661.8 \, \text{g} \end{aligned} $