What is the Big O estimate of a nested for loop?
The whole thing is $O(1)$. This is because the outermost loop has an upper bound of 100 i.e. it is constant, and it bounds the middle loop, which bounds the inner loop. There's a little bit of tricky math to figure out exactly how many times the loop is run, but you can know exactly how many times it is run.

The first hint that this is $O(1)$ is that there are no "free" variables. Pretend the code was linear. What would it be linear in terms of? There are no input parameters. We create $i$,$j$ and $k$, and their values don't change based on some input parameter.