When we consider the similarities between the third day and the Millennial Kingdom we should realize that, if flat Earth is Biblical, then maps of the supposed flat Earth should have Jerusalem at the center. They don't.
The third heaven is the dome or firmament, which is a sea of glass. It's a rigid crystalline sphere on the edge of space which formed in response to the creation of gravity on the second day.
Most of the space between the firmament and Earth is the cosmos or observable universe. There's something very odd about the cosmos: there's a disconnect between what we can see vs. what we believe about it.
The source of gravity for the observable universe is at the center of the Earth, it's an absolute frame of reference, and it's how we're able to use the Bible to deduce a unified theory of everything.
Axiom III is "Earth is the Center of the Observable Universe." We can deduce Principle I from the following physical evidence, empirical observations or established theories which are part of Axiom III.
The main evidence for the expansion of the universe is something called redshift. Redshift is the tendency for the light of far away objects to be composed of wavelengths at the red end of the spectrum.
The universe is both young and ancient simultaneously due of a phenomenon called gravitational time dilation (GTD). 2 Peter 3:8 is a mathematically accurate equation for GTD. We call it the Donkey's Jawbone.
Hell is at the source of Earth's gravitational field, it's iron-rich so it produces its magnetic field, and its rotation causes its electric charge (dipole moment).
On earth 1 day is equal to 1,000 years at the distance of the firmament, but at the firmament 1 day is equal to 1,000 years on earth. It's a paradox. It is also mathematically perfect in 2 Peter 3:8.
We use multiple lines of investigation including scripture, empirical observations, theoretical models, and physical evidence to deduce the presence of a sphere of crystal on the edge of space.