jhu 金数推荐信是几封学术啊？需要实习推荐信吗？（附官网要求）

官方要求
Arrange for three letters of recommendation from persons familiar with your abilities and achievements, especially relevant to graduate study in applied mathematics, to be submitted electronically through the online application.
这是三篇学术的意思吗？还是有实习推荐信也行？