I'm buying some 3DS consoles for my kids, 3 to be exact. If I buy Smash Bros for the 3DS digitally, will I be able to download that game to all 3 consoles or does the purchase only allow for download to one console? Will they be able to play together like this?
3DS downloads are tied to the console, so if you buy it once, it will only be valid for the 3DS it is downloaded to. If all of the kids want to play together, you will need multiple copies of the game (one per 3DS) for them to do so.
Unlike some games in the past, this game doesn't have any sort of download play that allows more than one 3DS to play using one cartridge/downloaded copy.
You cannot play between multiple consoles (3ds) with a single copy of the game.
Fortunately, there is a solution, but if you don't already have a Wii U, it will be substantially more expensive then buying them separate games.
If you buy the game for the Wii U, there is an in-game option to sync a 3ds with the game. If one of them has the game, he/she could play, but unless more than four people are playing, you might as well just play on the Wii U.
Basically, there is no good solution to your problem other than buying them all the game.