I am unable to figure out where to mark the floor on a tape measure. If I have 26.6 in how does this translate into 3 even sections?
It seems like your question is how to translate 26.6" into an accurate measure in fractions of an inch. The decimal equivalent of 5/8 is 0.625, so 26 5/8" is the closest fraction that'll be marked on your tape. If you mark out the two outer sections at 26 5/8, the middle section will be 26.55", or about 26 9/16", which is close enough for a bookcase.
That said, here's an easy way to divide a space of size m into n equal parts:
Mark the sides and front of the space to be divided on the floor.
Find xn, the nearest multiple of n that's larger than m on your tape measure. In your case, you're dividing 80 inches into 3 sections, so m=80 and n=3. The nearest multiple of 3 larger than 80 is 81, so xn=81.
Extend your tape measure and put the xn mark at a front corner of the space to be divided and the end of the tape (the zero point) lies somewhere on the line marking the opposite side of the space. Since 81 > 80, the tape is going to make a slight angle with the front of the space.
Put marks along the tape at the multiples of x, where x is of course nx/n. In your case, 81/3=27, so put marks along the tape at 27" and 54".
Place one side of a square (framing square, speed square, try square, doesn't matter) along the line marking the front edge front edge and the other side on each mark, and transfer the mark to the line.
The front line is now divided into n equal parts. If you also wanted to mark the centers of the sections, choose an xn that's also a multiple of 2, and then mark multiples of x/2. For example, let xn=84 (which is greater than 80 and also divisible by both 2 and 3). Then, mark all the multiples of x/2 (i.e. 14", 28", 42", 56", 70"). The even multiples of x/2 (28" and 56") are the boundaries, and the odd multiples of x/2 (14", 42", and 56") are the centers.