Generally speaking i3<<i7 (unless i7 is an Ultra Low Power version and i3 one is desktop chip)
Just a general rule in the naming of Intel chips, let's take i7-7500U
First letter after series name (i7 here) is 7, which denotes the generation of cpu. Higher the generation better the performance and power savings (So a 8th gen cpu will be better than a comparable 7th gen one eg. i7-8700K is better than i7-7700K).
After that those last 3 digits (500 here) defines the performance level of the chip (higher the number better the performance for comparable chips eg i7-7500U<i7-7600U).
Suffix in the end plays an important role, different characters have different meanings. eg. U- Ultra Low power chip suitable for notebooks and other portable devices.
For more read
here.