I don't really know of any way to slow it down. I have never had a problem with it being too fast to control or anything. I suppose if you somehow tied a knot in the tubing you could slow it but that might stop the syphon all together. I don't see what the problem with it coming out fast is. To syphon the dirt and waste out of the tank there has to be enough pressure to pick it out of the gravel.
Plug the end of it with a small rock. Obviously do not plug it completely, just enough to slow the flow. I always just use my finger over the end of it, the output that is, and then you can control how much the flow is. It can be akward if your bucket is a lot lower than your tank
Any time you decrease the flow it will pick up less dirt. Being a three gallon hopefully the water is being changed often enough that there would not be enough stuff at the bottom that you run out of water before you get all the scum
As a Syphon works on gravity, you can decrease the speed water passes through by making the drop different heights (i learned this making home brewed beer) by raising the collecting bucket/barrel/bowl up a foot or so, the water has less distance to fall and is slower.
Don't feel bad about having troubles with your siphon. Using them is not a skill you are born with---it's one you have to learn! It just takes a lot of practice to get it right.
You might try holding your thumb over part of the bottom end to slow it down, or try squeezing the hose part way down to to see if it will slow the flow down.
When you get used to siphoning you will be able to get the dirt sucked out faster, and that will help, too.
When you suck the dirt out, experiment with putting the siphon into the gravel and lifting it up at different speeds. Watch how much gravel is sucked into the tube and at what height it starts to drop back. Once you find out the optimum height and speed it will help.
Also, don't worry if some gravel gets sucked out into your bucket. Just try not to get too much and put it back into the tank. (It happens to me, too. ) )
dont u just hate that!! the other day i sucked up loadssssssssssssssssss of gravel because i took the end off my syphon... even sucked up a kuli loach who thought it was fun an did it again!!!! daft thing ended up in the bucket 3 times before he got the jist that i wanted the cr*p an not him, i ended up havin to put the end back on