New games are started via main menu (Game > New Game), the toolbar, or per short-cut ctrl+G. All parameters of the game have to be defined in advance, that means, amongst others, which board and what dictionary to be used. It's also possible to apply one of the individually defined presets. The dialog "New Game" offers the option how many players participate, and under which name or alias the players should be known in the game.
After starting the game the sequence is randomized. The players are supplied with seven tiles (or more or less depending on the configuration of letters). The goal is to place a valid word with the maximum score. The first player has to include the (gray) start square. The dictionary should contain the placed word and all combined, or other players agree on the validity. Rules explicitly allow all grammatical forms.
When the move has been finished, the computer restocks the rack, if enough tiles are left. Now the next player is on turn and tries to score with a nice word. Basically it's possible to build a new word but already placed can be extended too. However, at least a connection to existing tiles is required. The score is calculated for new tiles and already placed whereas premium squares are not included again in the calculation. If all tiles are played at once, known as Bingo, you get a special bonus), usually 50 points.
If a player cannot place any tile or does not want to for tactical reasons, he or she may pass or exchange letters (some regional rules limit the exchange to a certain number of tiles to be left). If the option "Cambio Secco" is active, you are allowed to exchange all letters without loss of the move, but only once the game. Other special rules permit to exchange placed jokers (blank tiles) or even to swap a tile on the rack into a blank tile ("jokerization" or "what-if variant").
The game ends when either one player plays every tile on his or her rack, and no tiles remain in the bag (regardless of the tiles on his or her opponent's rack), or after two successive scoreless turns by all players (the number of passes is optional). The points of remaining tiles are summed up at the end, subtracted from the own result, and added to the players result who ended the game. Winner of the game is the player with the highest score after this calculation.
Scrabble3D offers the option to play with a time limit per game as known in tournaments, but a limitation per move is also possible. And you can decide whether or not the game is lost after timeout, or if additional time is "bought" for points, for example one further minute for 10 points.