It means that there must be my teacher among the threesome. I choose his good aspects to learn from him, and correct my shortcomings against myself when I see his bad aspects.
Being quick and eager to learn is not ashamed to ask questions
It means that you are smart and eager to learn, and you are not ashamed to ask people with lower status and less knowledge than yourself. This sentence shows that if you want to gain true knowledge, you need to be eager to learn and kind to ask questions, and to do these two things, you need to have a modest attitude.
an upright man is open and poised and a petty man is anxious and worried.
It means that virtuous people are flat and broad-minded, while those without virtue are often embarrassed and depressed.