Maradona was the captain of the team, and since he arrived, the team won three championships in a row.

in 是介词，表示"在...之中"或"在...期间"。 a 是不定冠词，表示"一个"或"某一"。 row 是名词，意思是"排、行、列"。 in a row 是一个固定短语，字面意思是"排成一排"，引申为"连续地、不间断地"，表示一系列事件接连发生。