"文凭"usually represent a person's degree of education,when you graduate from school,you will get it."奖状" always use to encourage students who get good grades or behaved well at school,it is useless for us to find a job,and it generally used in primary school and middle school,"证书"represent your professional ability,it always need to pass some tests or win the game if you want to get it,like nutritionist certificate,it always connect to our job or special skill. I hope this can help you.