It depends if you are talking about prepositions of TIME or PLACE.There are also certain verbs that take one preposition or the other. For- can be for length of time."I have been waiting for an hour". AT- for a location "He is at school now". TO - for direction of movement "I am walking to school". But these words have many,many more uses.

I was at the store earlier. as for me I don't like shopping they are two very very common words and both have many ways they can be used. at this point in time, I think I will not go shopping for anything. I was at a point in my life, that going for an interview was very stressful for me. where is it at? what on earth where you doing that for .