I just got three goldfish for my grandon. The first few days they were swimming and fine. Now they seem to be sleeping in a bottom corner of the tank, they do swimming around when feed but return to the bottom together after eating. What am I doing wrong?

What size tank do you have these 3 fish in?  Common standard is 10 gallons per fish.  Regarless how small the fish are now they will grow.

do you have the tank filtered?  You should be running a filter that can turn over the water volume of the tank 10 times per hour.

And contrary to what asorvia said Goldfish are not lazy fish.  They should never be sitting still at the bottom of the tank.  Even when they sleep they don't "sit" on the tank floor.

I suggest doing at least a 50% water change making sure you use water conditioner to remove chlorine from the water.  In my experience it sounds as if your fish are suffering from what is called new tank syndrome.  Fish give off amonia from their waste and from their gills.  In a properly cycled tank (nitrogen cycle) your tank needs to age in order to grow good bacterias.  At first you will have a great deal of Amonia in your tank (reduce to safe levels by doing water changes) then there will be a bacteria that grows that will feed on amonia and give off nitrites.  Reude the nitrites by doing water changes.  Eventually their will be a bacteria in the tank that changes nitrite sto nitrates.  Your fish will be safe then from Amonia and nitrite poisoning, but you will still need to preform water changes to keep teh nitrates under control.  New tank syndrom is when there is much to much amonia and it is killing off the bacteria that needs to grow to convert the amonia to nitrites ect.  More frequent water changes are a must.  You should also invest in a good water testing kit so you know when your cycle is at which stage and also if there is any bump in the cycle.

Best of luck witht he goldfish!