  Low-metallicity, compact starburst galaxies referred to as Green Peas (GPs)
provide a unique window to study galactic evolution across cosmic epochs. In
this work, we present new deep optical spectra for three GPs from OSIRIS at the
10m Gran Telescopio Canarias (GTC), which are studied using a state-of-the-art
methodology. A stellar population synthesis is conducted with 1098 spectral
templates. The methodology succeeds at characterising stellar populations from
0.5 Myrs to 10 Gyrs. The light distribution shows a large red excess from a
single population with $log\left(age\right) > 8.5yr$ in the GP sample analysed.
This points towards an incomplete characterisation of the gas luminosity, whose
continuum already accounts between $7.4\%$ and $27.6\%$ in the galaxy sample.
The emission spectra are fitted with the largest Bayesian chemical model
consisting of a electron temperature, a electron density, the logarithmic
extinction coefficient and eleven ionic species under the direct method
paradigm. Additionally, building on previous work, we propose a neural networks
sampler to constrain the effective temperature and ionization parameter of each
source from photoionization model grids. Finally, we combine both methodologies
into a 16-dimensional model, which for the first time, simultaneously explores
the direct method and photoionization parameter spaces. Both techniques
consistently indicate a low metallicity gas,
$7.76<12+log\left(\frac{O}{H}\right)<8.04$, ionized by strong radiation fields,
in agreement with previous works.
