Burn injuries are rare but time-critical incidents in pre-hospital emergency medical services (EMS). Their assessment requires estimating the burned total body surface area (TBSA-B), which informs treatment, transport, and referral decisions. While three-dimensional (3D) approaches may support more precise burn documentation, they can also increase interaction complexity in a demanding work environment. We address this challenge from a human-centered design perspective. Based on focus groups, we derived requirements, developed a 3D prototype, and conducted a pilot evaluation with six EMS experts. Participants perceived the 3D prototype as a promising form of support, particularly for documentation, visualization of complex burn patterns, and decision support. Interestingly, they emphasized that the possibility of precise annotation increased documentation effort. Our work highlights the design challenge of supporting sufficient accuracy without encouraging unnecessary detail, contributing early insights for 3D documentation support.