Difference between i3 and dual core

i3 is better then Intel atom or dual core because a standard processor has one core and i3 has 2 cores, but An i7 processor has 2, 4 or 6 cores (depending on the processor model)

so, I recommended you that you must buy i7 because i7 is to much faster then i3.

If you will be working with CAD files, get an i5 or an i7. i3 is OK but not great for high-end work.