Help, my parakeet seems ill

Hello, long story short my bird was on the ground of the cage this morning. Doesn't seem to eat that much, doesn't sing anymore. It seems like he's sleeping (yes he's a male).

I originally had 3, 2 females and him, I went to a pet shop and the guy told me it's not recommended to have it like that and it's better to even the pairs (get another male), but I'm a total beginner, so I still don't know if it's true. He gave me a medicine that you're supposed to pour in the water to make him drink it. All I'm saying is from my visit 1 hour ago.

I just wanted to talk this out with people that might know before doing something wrong, but I want to act quick, my bird's seems I'll.

Im starting to read on how to take care of them properly and keep them healthy, please help I'm so worried

If at all possible, take him to a certified avian vet- not pet shop. When they are on the cage floor if they are not playing it is an emergency.
Meds in the water is never recommended by an avian vet because you cannot tell how much they are getting or if other birds are getting it...and then it has to be dumped out daily along with the water supply (as the dish needs to be cleaned everyday).

If this guy is all you've got, find out what he gave you and why. Without blood work or a gram stain he is guessing and you need to know what he thinks the bird has. This may give you some insight, but I am not a fan of some of the stuff he told you. I get why he said no females, but not sure why he felt males were necessary. At the same time, if he is the only resource within 5 hours, then asking questions could lead you to answers (with the help of Google, his opinion and the help of others online).

Getting a partner for a bird is an easy way to spread illness among birds unless quarantine procedures were in place.
A lone parakeet can be fine. 2 males may or may not get along (male and female will mate and that is a whole other health risk).

Try to keep him calm and warm (NOT HOT)...make sure he is eating and drinking and get him to a vet asap. Make sure you do not introduce any new birds because he has something that could be spread to them.

Also-- make sure you are not using any teflon or chemicals in the home..no candles, vaping, smoke, perfumes, incense, cleaners, sprays etc...All of these can kill birds---same with melting plastic, self-cleaning ovens etc etc.

So sorry you have a sick bird. Do you still have the others? Is he suddenly alone after having freinds? They can get sad with sudden loss if freinds
https://www.littlecrittersvet.com/sick-birds.pml
https://www.birdclinic.net/avian4.htm

https://lafeber.com/pet-birds/bird-behavior/