This is by far the stupidest possible way of handling this event. You're actually fucking with us right? This world almost certainly has slavery magic so voluntarily letting your strongest fighting force be enslaved is exceptionally stupid. Unless their is no slave magic here, my bet is that all three of the FMLs are permanently enslaved for the rest of the story because JP web novel writers have a weird obsession with making sure at least some of the MC's partners are their slaves.