in terms of hardware -
a6 3500 has 3 cores running at 2.1ghz clockrate with 2.4 ghz turbo. core i3 cpus have 2 hyperthreaded cores (2 threads per core) with various clockrate, core i5 usually has 4 cores and turbo boost (an auto oc feature, lets the cpu oc itself if thermal headroom is available). amd and intel implement turbo differently.
in terms of performance -
depends on what you want to do with the cpu/apu.
for general purpose tasks like web browsing and video watching, a6, i3, i5 are all okay. even a6 will do.
a6's integrated gpu is better than i3 and i5's integrated gpu.
a6's cpu side is less powerful than core i3/i5 processors. if you're doing processor intensive tasks, i3/i5 will perform better than the a6.
for gaming with powerful discreet gfx cards, i3 and i5 are better than a6. a6 is decent for low resolution casual gaming.
bottom line, a6 and core i-processors are not equal. they're quite different from each other.