simply 맵다 "is hot, spicy", 매콤하다 has the same meaning but slightly has a nuance of "less spicy" than 맵다. 얼큰하다 means also same spicy. In definition of the word, it says so hot, that my mouth's got numb. However, it may depend on people who use these words how much they think over each word relating to the nuance. For me, I might say 얼큰하다 when a dish is hot, normaly soup, and stimulates my nose for the spiciness or makes me warm inside :P 맵다 is commonly and mostly used word for the spiciness of food, however hot they are :)