Overall the i3 is a more powerful CPU despite having only two cores. But if you use programs that can actually all 4 cores (like video encoding), then the A8 would probably pull out ahead. However, which Core i3 are you asking about?
The i3-2370m is the older Sandy Bridge and has the Intel HD 3000 iGPU. The i3-3110m is the current Ivy Bridge and has the Intel HD 4000 iGPU which is basically 35% more powerful than the HD 3000.
The A8-4500m has the Radeon HD 7640g graphics core. According to the following link, in 6 out of 9 games benchmarked the Intel HD 4000 had better performance. I think for 2 of the benchmarks the Intel HD 4000 was only faster by 1% - 3%.