i would only put one male betta for sure and even if you decide to do a female betta instead, i would still only do one. Members have done femal betta sorority tanks but those have been much larger than a 3 gallon. As for shrimps, i suppose you could do a few ghost shrimps but even at that i still think 3 gallons is a bit close quarters.
There is nothing wrong with a 3 gallon tank and if you do it right you can have a very pretty looking tank...as long as you do things tactfully.
If you are looking to do more with a tank then i would get something a bit larger. I personally am not a fan of these "kits" i personally like to mix and match my parts since i can't get everything i like and want in one grande all, get all package. If you are looking to do something with a male betta, a 10 gal with some plants, low-med light wouldnt be bad since they dont really need CO2 and with that you could add a variety of shrimp and even some other fish mates. Neons in a group of 6-10 wouldnt be half bad along with shrimp and a betta. There are some other fish you could add as well, i find tetras to be pretty docile and will school if there is a "threat" in the tank.
Corrieberry made a great point above, follow the suggestions given there. Above all, maintainence of the tank is a big big deal. Bettas can endure really crappy water but you know what, they shouldnt have to, there is no reason.
keep in mind, i found that my betta struggled alot with current within the tankand this seems to be a very common occurence especially since bettas are commonly found in very slow moving water sources in thailand such as rice pattys. I always had to keep my HOB filter on low so as not to bother him, i even had plants and rocks and a castle, he loved the castle, he always slept in it.