You should not add fish to the tank like that, meaning don't put them in the tank and then bring the home. Never buy fish at the same time as the tank.
You should get the tank home, set it up and fill it with water. Then you should go back 2 days later to get fish once the tank has settled. You bring the fish home in a bag, float the bag in the tank for 10 minutes, and then put the fish in.
As far as cold water fish go - NO GOLDFISH! Goldfish are huge carp and need ponds and your tank will kill them in no time at all. You can get 3 white cloud mountain minnows or 3 rosy red minnows for a your 9 litre tank and they will get by, if not thrive, if you take care of it properly.
You need an air pump at least. without water movement the water becomes stagnant and stale, and can become deprived of oxygen. A filter is better, but an air pump with an air stone will do for this little tank.
Remember, if you don't clean the tank every week (draining and replacing half the water and vacuuming up the gravel) , you are not taking good care of the tank and the fish will decline in health. Also feed them just a few flakes each per day - do not dump food into the tank. There should be not one single piece of uneaten food in the tank when you are done feeding. Overfeeding causes fat, unhealthy fish and dirty tanks.
The others are all correct about the size of your tank and how unsuitable it is for fish because they like to be in groups and need space to swim around, and water can't be maintained properly in tiny containers. The rest of my advice is just assuming you have no other choice, can't go any bigger, and must make do.