The present study aimed to determine the chemical composition and the synergistic effect of three plants’ essential oils (EOs), Eucalyptus camaldulensis (ECEO), Mentha pulegium (MPEO), and Rosmarinus officinalis (ROEO), against three bacterial strains, Proteus mirabilis, Klebsiella pneumoniae, and Staphylococcus aureus, in order to increase the antimicrobial effectiveness by the use of a low dose of essential oils, consequently decreasing the toxicity and negative impact. For this reason, an augmented simplex-centroid mixture design was used to build polynomial models in order to highlight the synergy between the essential oils against bacterial strains. Antimicrobial effect screening was performed by the disc diffusion method and the minimal inhibitory concentrations (MIC) were also studied. The gas chromatography/mass spectrometry (GC-MS) results show the richness of these essential oils by terpenic compounds, especially 1,8-Cineole and P-Cymene for ECEO, Pulegone for MPEO, and α -Pinene and Camphene for ROEO. Moreover, a significant antibacterial effect has been demonstrated and the best values were revealed by MPEO and ECEO against P. mirabilis and K. pneumoniae, with inhibition zones (IZ) of 25 and 20 mm, respectively, and an MIC of 0.0391% (v:v) against K. pneumoniae. The optimal mixtures showed a synergistic effect of essential oils, and the lowest minimal inhibitory concentrations of the mixtures (MICm) were in the order of 29.38% of MPEO, 45.37% of ECEO, and 25.25% of ROEO against P. mirabilis and in the order of 60.61% of MPEO and 39.39% of ROEO against K. pneumoniae. These results indicate the antibacterial efficacy of the three essential oils combined and suggest their importance in the treatment of urinary tract infections caused by resistant bacterial strains.