Here's the advice you didn't want to hear:
In my experience, it's extremely difficult maintaining steady temperatures in bowls that small. I've used some really small heaters before and they tend to overheat the water when they kick on.
If you're concerned about cost: A ten gallon tank costs $11 at Petsmart, and you could get a decent heater for it for about $15. Place two dividers in the tank, use the decor you've already got in the bowls for each third of the tank, and presto. The larger volume will be much more forgiving, and it will cost less than the price of three of those micro heaters. A ten gallon tank only has a 20" by 10" footprint.