Sorry, but before they can... I need to give a choice. Sorry. Namely, what type of alien, or alien "something" is it?
Is it a Symbiote? Something like "Venom", that attaches to a body, more-or-less possesses people, etcetera etcetera, I mean. And, is it a seperate entity? Or does it turn one of the trio into one? For instance, Charlotte becomes possessed by an alien thing vs. Charlotte becoming an alien hybrid thing and possessing others. Obviously, it doesn't have to be Charlotte though.
Is it a Vacationer? It could look like whatever, depends on what you like, and its (or their, if there's multiple) whole thing is simple. It wants a vacation, and human bodies make great "getaways". It could be a consensual thing with the three, maybe there's an advantage to getting possessed by the alien. Or, maybe it's definitely not, and the three don't really get a choice. Up to you.
How 'bout an Invader?" Once again, it could look like whatever. Slime, something slug-like, maybe a little goblin-looking thing, idk. But their "or its, idk how many there would be) whole thing would be invasion, and conquest. Of course, with human hosts as well. Maybe the aliens can change their hosts, permanently or not, as well. So, for example, maybe the aliens want their bodies to be taller and muscular, or... maybe hermaphrodites. The reason exactly? It's up to you.
How about a Breeder? Basically, these aliens need humans to breed. Whether that'd be semen, saliva, whatever, it's obviously easier to get this from inside a human. And whether they'd outright control the body or just influence it... that's up to you. Hell, maybe it's consensual with people because they produce a "love pheromone" or something? I don't know, but it might make someone like Charlotte more willing to be influenced, if not outright hijacked, if it'll help her get a little action or something.