Need to reference the value of a third column when searching a range for a specific value

I have a large amount of data (50,000 rows or so), and for the sake of my question there are 3 relevant columns; column A that contains a program number, column B that contains a task number (there are many tasks that will have the same program number), and column C that contains task numbers that have met a series of other requirements (all these task numbers exist in column B). I'm trying to write a function that when i search column B with a specific value from column C, it will give me the value of column A where column B is equal to that value from column C.
Example:

Column A...........Column B............Column C
Program#....... Task#..............Important task#.........Output that i want
1...................... A ......................... B................................1
1...................... B ......................... E................................2
1......................C .........................H..................................3
2......................D .........................J ..................................3
2...................... E .........................M................................ 4
2......................F
2...................... G
3...................... H
3...................... I
3...................... J
4...................... K
4...................... L
4...................... M

I understand the concepts of writing functions and I've tried tinkering with making an IF function work, but I don't have a big enough knowledge base of them and I just can't seem to figure out how to make this work or if it's even possible. I'm open to any sort of solution that will accomplish the end goal. Thanks!