Tactics is the science and art of handling troops and military units in the presence of the enemy. By extension, it can mean any maneuvering or adroit device to accomplish a goal. The adjective form is tactical.
As an illustration, the military term tactical unit refers to the largest group of troops or troops and equipment that can be directed by a single officer.
Strategy is the study or science of military position on a broad scale for winning in a war. It can also refer to the use of stratagems in business or political affairs. The adjective form is strategic.
Stratagem is a maneuver designed to deceive and outwit an enemy in war. By extension, it can refer to any action designed for obtaining an advantage.