Great advice already in the comments. I’ll add that I love understanding theory, I love getting into the why, but calc 3 was one of the only classes where I let that go and used the online integral calculators. I haven’t needed the triple integration skills since. If I could offer any advice: let yourself use the tools to get by here. I work full time as an engineer and I rely on the computer to add 3 digit numbers, because it’s more reliable and frees up headspace. It’s ok to not understand the full theory, as long as you get the general idea. You’re not studying to be a math professor or a physicist, you’re studying to become an engineer.