The first question when anybody hears about the Intel Core i3, i5, and i7 is the numbers. Why three, five, and seven? Do the numbers actually signify anything? The numbers are not about specific technical features like the number of cores. Instead, as you can see in the processor star rating definition screenshot, it is about the low-end (i3), mid-range (i5), and high-end (i7) which correspond to the three, four, and five stars. Two and one stars are represented by Pentium and entry-level Celeron processors.
This still leaves us with the question: why three, five, and seven? As we have already discussed, they correspond to three, four and five stars on the fabulous Processor Star Rating Definitions chart (see screenshot).
Here is my conjecture: They didn't want to use four and five because they do not want customers to see i5 and i7 as small steps up from the i3. I'd imagine marketing at Intel would like the product segmentation to be a bit clearer. Thus, the need to put some difference between three and four stars as well as four and five stars. It is my guess that the i3, i5, and i7 nomenclature has its reasons based on marketing.