Everyone does it differently. I don't even monitor humidity or add water in the last week of incubation... however that is how I got my set up to work.
Ideally you get a set up for your single egg ready and up the humidity for that last 3 days. You can do it in the regular bator, but you do risk the eggs already in there. Now will it kill them all, unlikely, but it does increase the risk. You could just not increase the humidity and the silkie eggs will be fine, but you increase the risk to your one egg getting stuck.
It is a balance between the two and there is no one hard fast rules on how you have to do it.
Unless you are experienced, get a second set up at the standard 70% humidity for last three days for your test egg so you don't risk drowning the silkies.
Edit: In addition, if your chick does hatch out in yoru bator with the rest, it will leave some hatching goo on things and that hatching goo very quickly starts to grow bacteria which you don't want to have when your others hatch out 5 days later.