Before you get too excited remember this:
A single female ADF needs about three gallons, and a male needs two and a half MINIMUM!
They do great on sinking pellets and frozen (thawed) blood worms, and all of my bettas (from CTs to PKs) have gotten along with them no problem.
At the least, if you only want one frog and a betta, i would get a five gallon tank, the three gallon tank that you have is much too small for a frog, they like to move around quite a bit in the right conditions.
A sand substrate is best, and the bigger the grain that you get, up to pebbles, rocks and gems, the worse it will be for your frogs. They have very delicate toes and legs, and getting pinned under a rock or gem could be fatal.
Males do sing when they reach the proper age if you have several frogs, more boys than girls, but the singing is to attract a mate, so be careful.
As long as you properly QT your frog, and make sure he/she is healthy and eating well, they will be a great addition to the right tank.
Also, personally, i find that not feeding them in the same place every time is best, it gets them moving, hunting, and keeps them active. I drop three pellets per frog in random places in the tank at different times of the day, once a day, and it is more natural. I also feed blood worms to everyone every other day.
Hope i helped, and if you have any more questions, feel free to pm me. :)