They both have the same core/thread count with 4 cores and 4 threads. The 8th generation i3 is newer and has a number of architectural improvements over the 3rd generation processors making probably about 15-20% faster clock for clock, and the i3 would be clocked higher than a stock clocked i5 from 3rd gen. About the only drawback an 8th gen i3 has is it can't be overclocked, so a 3570k might be able to close the gap somewhat if you overclock it.
Other than that the 8th gen i3 does have advantages to being on a more modern platform eg. DDR4 RAM support, M.2 support for high speed SSDs.
If you already have a 3rd gen i5 CPU and are asking about upgrade options, you'd probably want to go a bit higher end than the i3, 4 cores and 4...