One strip wood flooring, as the name suggests is wood flooring that comes in the style of one strip of wood. In other words, it has one, single strip of solid wood that goes from one side to the other. On each floorboard you have no joins mid-board, all you have is the edges of the board
Three strip wood flooring, on the other hand is floorboards that are made to look as if they have three floorboards in one. They may be as wide as 45 or 60 cm, or even wider and will have a solid wood top layer, but that solid wood will either have been made to look as if it three separate pieces of hardwood or it may indeed be 3 separate pieces.