3 weeks for a 6' boa definitely should not be enough time for it to get hungry, my large and mature boas only eat every 4-6 weeks. That said, I would not feed the snake at least another 1-2 weeks before leaving. This will reduce the chances of defecation, and also if anything does happen (like extreme temps), you don't want food in their stomach. I went away for a week and had someone watch my snakes...came home to the room over 100F, and the only one that didn't make it or return to normal was the one the babysitter fed either twice or three times - idr - in my absence despite explicit directions not to feed any of them. No food should be digesting in their stomach when you leave, this is extremely important.

So, finding someone competent just to keep an eye on temps and water is also extremely important. They need to be able to at the very least change water, if not clean the cage if it gets messy. I also like the camera idea pitched by a few people! This will allow you to keep an eye on the people/person watching the snakes, because theft is not unheard of in the reptile world. But, it can also help give you guidance to the caregiver.