Poly(3-hydroxybutyrate) (PHB) ternary nanocomposites were obtained by solvent-casting method. The current work compares the behavior of nanocomposites made with two different plasticizers and two types of nanocellulose (cellulose nanocrystalsCNCand bacterial celluloseBC). A traditional plasticizer (glyceryl tributyrateTB) and a polymeric one (poly[di(ethylene glycol) adipate]A) were chosen in order to evaluate the responses of PHB properties to the addition of plasticizers with different molecular weight. Additionally, the influences of CNC and BC on the properties of the plasticized polymer were compared. Cellulose nanocrystals were determined as effective PHB nucleation agents when TB was added to the samples. On the other hand, the addition of A and CNC seems to retard PHB nucleation but enhances the thermal stability of the samples due to the intrinsic stability of the additives. A counterbalanced effect of TB plasticizer and CNC was observed in the mechanical and water vapor permeation properties. The addition of BC to the plasticized samples showed the highest detriment in thermal stability of PHB due to its poor dispersion. It was concluded that PHB/TB/CNC nanocomposites will be suitable materials for using as packaging in single-use applications, showing an appropriate balance among barrier, thermal, and mechanical properties.