I have some reservations about 2 bettas in a 3g tank. The general guide is 1.5gal=1fish. Now it seems like 3g would be plenty but you have to consider all the other stuff in the tank and how much water is being displaced. So a 3g with a couple of plants, rock or driftwood shrinks the water volume down.
Generally, 1 male betta should be ok in a larger community as long as you pay attention to his tank-mates; no nippy, aggressive fish, no competitors for betta's food and no larger-than-betta fish. Additionally, you have to pay attention to where
fish generally live in a tank. Bettas make use of their whole tank. So 1 bottom live-r may be alright, i.e. plecostamous.(spelling?)
Having said all that, if your male and female are content together, and especially if the male is not chasing her, terrorizing her, try it. However, assuming they may spawn someday, you will have to take the female out within 1-3 days because the male will
chase her away from the bubble-nest. Male betta will have to be removed as soon as the fry are able to swim around on their own in another 1-3 days. So in the end, your economical 3g will not be enough and you will have to purchase 2 other places for your adults. 1 (3g)x 3=3 fish containers.
My suggestion would be to buy a 5-10g, let them live together, all above concerns being addressed. Get a divider for eventual spawning and keep the 3g as a quarantine.
With regard to plants; get real ones. They help maintain the bio-environment, naturally, less work for you, healthier for your fish. I bought mine from Aquariumplants.com. The plants were shipped 2day and all arrived in good condition and are acclimating well, except one. However; Aquariumplants.com
has a strict policy regarding guarantees; if you choose not to overnight the plants, no guarantee.
Edit: Tank temp; 72 Fahrenheit min constant temp, can fluctuate a degree or two down but not for more than a couple of hours. Ideal temp range is 74-82 Fahrenheit.
If your house temp fluctuates up and down, set your temp to the 76-ish range to be safe.
Additionally, you need to be testing your water
ph, ammonia, nitrIte and nitrAte levels to insure good water quality. All this stuff can sound intimidating, but once you get the initial conditions stable and within healthy range, the daily work-load goes down, drastically.
You can see my 10g cycling photo-log by clicking on my link. Keep us updated, Good Luck!
Crap, one more thing. About those outdoor rocks, good for you for sanitizing them. I assume there are no jagged edges to tear fins. It is not generally a great idea to use any old rock. The chemical content of differing types of rocks may leech into your water. Copper=no good for bettas, kill 'em fast.
I do a little metal work as a hobby, I have to be very careful to get all the copper off my hands before handling any of my fish stuff.
Iron, no good either.
If I have made you feel badly, please know that is not my intention. My intention and the intention of all the fish-keepers here is to help you become an expert fish-keeper; then you can pass along your expertise to us!