How to check if multiple conditions exist and give 1 of 3 answers

length phase duration (months)
12 1 5 6
18 2 6 10
24 3 8 13
30 4 10 16
36 5 10 21
42 5 13 24
48 6 15 27
60 8 18 34
phase>>> 1 2 3

I'm not sure this is where I should post this, but I can't figure out how to do what I need to do!

I have clients who are enrolled in programs of varying lengths of time (length=12 to 60 months). Each client is in a 'phase' based on the number of months they have been enrolled. What I want to do is create a formula (I think) that will automatically give me the current phase that the client is in. For example: I want column X to display either 1,2 or 3. If the client is in a 36 month program and they have been enrolled for 5 months or less, then they are in phase 1. If that client has been enrolled for more than 5 months but <=10 months, then they are in phase 2. I know that I have 2 conditions, in 2 different cells that must exist before I can give an answer in a 3rd cell. I am lost.
I currently use the datedif formula to determine how many months they've been enrolled and how many months they have left in their respective program. That information is spread out over 3 columns, but I need a fourth column that displays the current phase my client is in. I have been entering it in manually but I know there must be a way to do it automatically.

Here's a way using Named Ranges.