Using the categorical approach to Poincaré-Birkhoff-Witt type theorems from our previous work with Tamaroff, we prove three such theorems: for universal enveloping Rota-Baxter algebras of tridendriform algebras, for universal enveloping Rota-Baxter Lie algebras of post-Lie algebras, and for universal enveloping tridendriform algebras of post-Lie algebras. Similar results, though without functoriality of the PBW isomorphisms, were recently obtained by Gubarev. Our methods are completely different and mainly rely on methods of rewriting theory for shuffle operads.2010 Mathematics Subject Classification. 17B35 (Primary), 16B50, 18D50, 68Q42 (Secondary). 1 It is perhaps worth mentioning that we do not consider in this paper another important kind of universal enveloping algebras of post-Lie algebras, the so called D-algebras prominent in the Lie-Butcher calculus [11,20]. Those algebras are obtained via an adjunction not arising from change of algebraic structure and, as a consequence, are a bit different; a PBW type theorem for them is true on the nose because of the Lie-theoretic nature of the definition.Acknowledgements. I am grateful to Vsevolod Gubarev for a discussion of results of [14], and to Murray Bremner for his comments on the paper [9], and particularly for a query as to how results of that paper may be applied to post-Lie algebras.