入伍 is a separable verb (离合词) that means 'to enlist in the military'. The verb 入 and the object 伍 (archaic term for a military unit of five soldiers) can split apart when adding aspect markers, measure words, or other elements — for example, 入过伍 ('has enlisted before'), 入了伍 ('enlisted'). Unlike a regular verb, you cannot put a duration or number directly after 入伍; instead, insert it between the syllables: 入伍三年 sounds wrong — you should say 入了三年伍 or 入伍三年 (though the latter is sometimes heard, the separable form is more standard).