Applying the combination of the master and stochastic differential equations, we investigate in detail a continuous-variable entanglement of the twin beam generated by the coherent beat laser containing a parametric amplifier and coupled to thermal light of an external environment. The dipole-forbidden transition of the three-level atoms are coupled by the initial coherent superposition and classical pumping light emerging from the parametric oscillator. The atomic coherence induced by the classical pumping field and the initial coherent superposition induce a strong correlation between the two-mode radiation, which results in a high degree of the photon entanglement. In addition, the parametric amplifier enhances the achievable degree of entanglement of the two-mode fields. On the other hand, thermal light appears to degrade entanglement but a strong photon entanglement can be generated by managing the amount of thermal noise entering into the laser cavity through the output mirror.