The 3-4-3 is a soccer formation that consists of three defenders, four midfielders, three strikers (forwards), and a goal keeper (not mentioned in the name). This is the formation generally used in the leagues of younger kids (Under-12 leagues), and is not uncommon up through about the Under-15 leagues. It contains the same basic positions as the 3-5-2, and these positions have the same general duties as in the other formation as well.
The purpose of the 3-4-3 is as a formation for less experienced players who do not have the tactical knowledge, nor the ball handling abilies to put balls in the back of the net with only two strikers. Also, at these age levels the two wings cannot always be counted on to support the offense as well as would be needed with only two strikers.
The midfield in this formation may have an offensive and defensive midfielder, but it is not set in stone. One will usually play about five to ten yards ahead of the other who will stay behind and at an angle. The center midfield is like a piston; when one player pushes up on offense, the player on the other side drops back to cover the open space. Sometimes the four middle players will be set like a diamond instead of a line, so that one midfielder does act as an offensive midfielder, and the midfielder in the back hangs behind the wings as a "stopper" who "stops" the passes that come through the middle.
The defense can either be a flat back system (as is often the case in the younger leagues) or have a sweeper, who stays behind the main two defenders to "sweep" up whatever gets through.