The verb dormire "to sleep" is in another group than whichever verb you took the -ant ending from. There are 4 different groups. You could look up an overview of verb inflections, just look at the present tense for now. See if you can match other verbs you've learned to one of the four groups! Good luck!
'The three sons have not slept' would probably be translated tres filii non dormiverunt.
'The three sons will not sleep' probably something like tres filii non dormient. This is assuming you mean 'in the future they will not sleep' and not that they are not wanting to or able to sleep.