1) If you took one class from me and got a B, the letter is unlikely to be helpful to you (unless it is a B in a graduate class).
2) if you got an A or A-, you should still keep in mind that the essence of a good recommendation letter is a concrete comparison with other students. I am expected to disclose your percentile ranking in the class and describe the level of hardness of the class. Being in the upper 25% in Calc 3 certainly won't hurt you but all alone it is not going to get you into an Ivy League school. If you are unsure about your standing in a class, you are welcome to ask me for this information.
3) Unless I got to know you personally, I will not be able to write much besides this ranking information.
4) Remind me about deadlines.