**Core Concept**
The question requires the application of the Lund and Browder chart to estimate the total body surface area (TBSA) involved in a burn injury. This chart is a widely accepted method for estimating the TBSA burned in children.

**Why the Correct Answer is Right**
To calculate the TBSA involved, we need to refer to the Lund and Browder chart. The chart provides percentages of the total body surface area for each body part. For a 3-year-old child, the face, scalp, and circumferential burns around both thighs are critical. The face and scalp together account for approximately 18-20% of the TBSA, while the buttocks account for around 2-3% TBSA. The circumferential burns around both thighs would add up to around 10-12% TBSA. Adding these percentages together gives us a total TBSA involved of around 30-35%.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is likely too low, considering the involvement of the face, scalp, and circumferential burns around both thighs.
* **Option B:** This option is too high, considering the Lund and Browder chart estimates for these body parts in a 3-year-old child.
* **Option D:** This option is likely too high, considering the involvement of the face, scalp, and buttocks, which would not add up to more than 30-35% TBSA.

**Clinical Pearl / High-Yield Fact**
The Lund and Browder chart is a crucial tool in burn care for estimating the TBSA involved in a burn injury, which in turn helps in determining the severity of the burn and guiding treatment.

**Correct Answer:** B. 35%.