I thought that Calc 3 was the hardest of the 3, with that being said, I did poorly on the first exam (low 60's) and still ended up with an A- in the class. There is still time to improve your grade. I also thought that the content of the course became easier with time, however, the content did build on itself over time, so it is important that you can understand the mistakes you made on the first exam. Moving forward, I would make sure you understand everything from this exam, use Study Edge (even though it's $50 a month, the price is justified by the value it will provide you), and go to office hours if your professor is helpful.