2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/71938We construct a series of finitely presented semigroups. The centers of these semigroups encode uniquely up to rigid ambient isotopy in 3-space all non-oriented spatial graphs. This encoding is obtained by using three-page embeddings of graphs into the product of the line with the cone on three points. By exploiting three-page embeddings we introduce the notion of the three-page complexity for spatial graphs. This complexity satisfies the properties of finiteness and additivity under natural operations.32 pages with 9 figures, submitted to J.Knot Theory and RamGeometric Topology57M25, 57M15, 57M05text