And, what if I use RO water for now instead of treating the water with chemical? Will that be okay? Or should I be concerned that all the fries would turn out female? Since I heard that RO water is soft and...soft water is reluctant to give us gonopodiums?
Don't use RO, because it's soft water. Not about the development, but guppies are a hardwater fish - they've evolved to live in hard water with all the mineral content it contains, and over the long term, would struggle and fail in RO water.
Is a basic water conditioner not available where you live? As a forum on the whole, we don't like using too many chemicals in our tanks, but water conditioner is an exception since tapwater nearly everywhere is treated with chlorine to make it safe for humans to drink, and the chlorine is toxic to fish. API water conditioner and Seachem Prime are excellent brands that make the water safe, without a ton of extra chemicals or additives like aloe vera.
What water are you using for them at the moment?
Three gallons is on the small side... and any size container is a potential problem without a filter, since it's uncycled. You'll want to be doing large daily water changes on an unfiltered 3 gallon with fish in it, but large water changes with water containing chlorine is also a danger... You really need to think about your set up - especially with more and more fry coming. Where and how to house them all while they grow, and how to keep the tanks cycled and low in nitrates for them to be safe and healthy.
Are you relatively new to fishkeeping? Do you know what the cycle is? Not asking to be mean! Just want to help, so need to know what knowledge you need to keep the fish happy and healthy!