I have a regular computer with CORE I3, according to information here, it's said that it's very difficult to mine any coin.
Is it true?
You will barelly get a few cents and spend a lot more in energy just a i3. If you really want to mine altcoins and earn money, you will probably need to buy a few GPU's.
For example, if you mine at Nicehash with a i7-7700K (the one I have), you will end up earning only ~$40 per month (minus your energy bill). And that's not very profitable.
As mentioned above, you really wouldn't bother CPU mining. It'll make your CPU run full tilt and be bad for your hardware. If you want to get in to mining, source some GPUs. Even get a few cheap ones to begin and learn before investing more in the future on better equipment if you decide to continue down that path.
Good luck.