How to ask for time off for an internship designed to help me leave my job

I just got out of the military into my dream (for my credentials) job doing IT work consulting at a consulting company (PwC EY Business Consulting, Accenture, Booz Allem). I am applying for 2025 Investment Banking Internships because that's where I am headed but didn't have the credentials to even get an interview. I wanted to do what I'm doing here at an IB, or even in NY, but my wife wanted to stay here. She said she's ok with going pretty much anywhere else for up to 20 or 30 years but wants to stay where we are at for the next year. 

If I get accepted into a 2025 internship - how do I bring it up to my boss that I want to take 3 months off of work - unpaid of course - but want to return for (ideally) about one more year until I leave for good? I brought it up lightly during my internship here that I aim to go into finance when I finish undergrad, and he seemed super supportive. He mentioned that lateral movement within the firm is relatively easy, but I learned as I got my FT offer that I'm on a hard-to-fill position which means probably hard to lateral out of (they accepted my ask for a 30% bump over the posted TC), so I'm worried about ruffling up the wrong feathers.

If I do get an internship offer, I want to let my team know so they can prepare and guide me on what they need for a smooth handover to the next person, but I don't want to hurt my reputation at the firm. Am I an asshole for even considering this?