eg.: i can do that. can translate to 能，可以，and 会。 if u wanna tell some kind of ability u hv.we can translate it as "u 能 do that." if just wanna tell u know how to do that.we use 可以 and also 会. all depends the meaning u wanna express.

能means the abilityof doing something,it ususlly refers to something that a little difficult. e.g. Can you write withh our feet? Yes, i can(能) 可以means you choose one option,or you prefer one than another. e.g. Can you help me ?? Yes .(Under this condition ,you could refuse or accept his requirement,but you choose to help him.) 会means you have the ability to do something,but it is easy or even if you can not,it's normal !!! e.g. Can you speak English? Yes, i can (会) （It is similar with 能，a littele difference lies at the difficulty that the things you do!!） It's my own opinion, hope it can help you !!!

能 also means I CAN （我能）CAN YOU DO IT？你能吗？the point is that you can do it ,you have ability to do it. If someone ask you that,你会吗？He want to konw whether you know how to do it.

complex question

会 emphasis something subjective, but 能 including some objective element。 可以means OK usually。 for exampl : there is a assassin said to the senator ：of course ,i mastered会 more than 20 ways to kill the president,but there are too much cops ,agents and citizens round him in the speech , so it can't be done不能。but if you gave me 10millions ,with a good preparing, the cops ，agents would be nothing。 the senator: 10 millions ??OK.可以. the president worthy that 。