Should you buy top soil or potting soil. I have been told
Top soil lacks nutrients, which must be added? How long does it take for a
Three gallon mango tree to bear fruit?

Why would top soil lack nutrients? It is the perfect vehicle for adding to gardens; but it is also necessary to use a 'pick me up' like additional vitamins and additives.

Are you going to grow the mango tree inside, in a pot? Then use a good quality potting soil. As to when does it bear fruit, maybe never depending where you are.

I think potting soil has a few extra things to better hold water than regular topsoil will have. But there are a few different types if potting soild tailored to the type of plant.

In a pot... I'd go with the potting soil. But both would actually work...