I would go for a 25 watt heater for a 3 gallon tank. I am not sure about heaters available in your area sorry. I use Aqua One adjustable heaters and highly recommend adjustable ones. Hope someone else can help!
if its adjustable try it out, i bet you can make it work, i have a 50w in a 3gal...
+1 make sure you test it out without the fish in the tank. You don't want to risk overheating the tank and killing your fish.... Or you could start with the lowest temp setting if it is adjustable and increase it slowly while checking the temp often. Give it a few hours each time until you get the right temp.
I use one of those 50W preset tetra ones for my moms bettas 3gal. It was my spare that I used in a quarantine for my endlers. My moms betta was in unfiltered, unheated water so I gave her my spare filter and heater, it works and keeps the 3gal temp at 78.
25 watt heaters are usually recommanded for 3 gallon tanks. You could technically use a 50 watt, but you are running a risk if it malfuntions.
I personally use Eheim Jager 25 watt adjustable heaters in all my tanks. They are very reliable, but they are larger than most heaters.
I also heard and read good reviews on the following;
Cobalt Aquatics NEO-Therm 25 watt
Fluval Marina Mini 25 watt
Visi-Therm 25 watt
Reading reviews and reseaching can save you the headache longterm. I found out the hard way, that all heaters are not created equal.