Three-dimensional integrated circuits (3D ICs) have emerged as a promising solution to the scaling limits of two-dimensional designs, offering higher integration density, shorter interconnects, and improved performance. As design complexity increases, effective space planning becomes essential to reduce dead space and ensure layout quality. This study investigates the use of large language models (LLMs) for 3D IC space planning through a post-order slicing tree representation, which guarantees legal space plans while aiming to minimize dead space. Open-source LLMs were fine-tuned on large-scale synthetic datasets and further evaluated on MCNC-derived 3D benchmarks. Experimental results indicate that the proposed framework achieves a favorable balance between runtime efficiency, legality, and dead-space reduction, with zero-dead-space layouts obtained in a significant portion of test cases under practical runtime budgets. Beyond synthetic benchmarks, the method generalizes to MCNC cases such as ami33 and ami49, though larger and irregular instances remain challenging. The approach also shows potential for cross-domain applications, including logistics and 3D object placement, where spatial efficiency is critical. Overall, the results suggest that LLM-based space planning can serve as a data-driven complement to traditional electronic design automation (EDA) methods, providing new insights for scalable 3D layout generation.