When they say the first and third bright fringe, do they mean m=1 and m=0..do we count m=0?
And likewise for when they say first and third dark fringe, is it m=1.5 and m=3.5?
How would we know when to count m=0?
The central bright fringe is the place when the path length form the two slits is zero.
The dark fringes on each side of this central fringe is where the path lengths differ by half a wavelength.
The bright fringes outside of those is where the path lengths differ by one full wavelength.
And so on...
That's the physics of the thing.
Now what's this character m? Never heard of him.
Suggest you write out some equation where it appears and explain what it stands for.