K-means clustering (k = 3) of r-axSpA, PsA and SLE patients was applied to construct disease endotype clusters. Serum C1M, C3M, C4M, C6M, and PRO-C3 markers were elevated up to 4.2-fold (p <0.003) in r-axSpA, PsA and SLE patients versus healthy donors. This evidence concerns the gene C3 and systemic lupus erythematosus.