I only change underwear once every 3 days.
Why not everyday?  It's pretty gross, otherwise.
Is it really unhealthy?
Usually I would be able to defend your choice but not changing your underwear does bring the chance of bacterial infection. You're also subjecting people around you to that smell.
I'm not going anywhere for a few days so whatever.
Well even if no one else is around to be offended, I still hope that you will take my advice and be a bit more hygienic.  It can be a hassle to do more laundry, but it's not worth the risk to your health.