My 3 yr old is getting gelded today. How long till I can put him back to work? Any tips on getting him healed up? How long before the hormones die down?

Return to work all depends on the horse.  Usually just a couple weeks, but ask your vet.  As far as hormones, I gelded mine at age 4 last march and he still acts like a stud.  

I gelded a 5yo many years ago. He still wanted to break into his favorite mare's field for a year, but that started dying down after the year. He went back to work in about 3 weeks - but I took it slow with him and let him walk-trot at first and then worked back up to a full workout. This horse still poofed up at other stallions for many years even after losing complete interest in mares! He still hates Zan, and I have to keep them at least two fields apart still to this day. 

Physically, usually 2 weeks you can go back to light riding. Keeping flies away is the hardest part, I usually do mine in the cold months so there's no flies and the cold helps the inflammation. And just watch for anything abnormal during the healing, keep clean. For the mental aspect, that's going to completely depend on the horse and how "studdy" they were to start. I have one I gelded at 7 after he was a standing stallion and at 23 he's still a stallion mentally. He lives by himself, he will try to kill geldings and just jumps mares all the time. I had another I cut at 2.5  and he was never studdy at all, so there was no behavior change after. 

You can ask your vet about Depo, it will help with the hormones.  

The testerone levels drop really quickly after gelding, almost in half unfortunately learned behavior is another story entirely.  I gelded one at 5 and within 6 months he became easier to work and haul, over time he did improve in behavior.  I have used MVP Levelor on a gelding that was gelded late and was so frustrated with his behavior.  It actually worked really well.