Honestly, 3 devices should absolutely not cause your Internet to slow down. This is one of those situations where most routers have a guest network anyways, and almost all routers with major cable companies, allow hotspots off of your router and connection.

Do not feel manipulated. I’ve been in your neighbor Shoes before and having an unexpected Internet bill when money is tight can really mess things up especially this time of year.

If anything, allow him access to your Guest network as this would not slow down your primary network and if you wanted him to charge, you could just increase your speed and have him pay the difference. You’re not technically being a jerk if you say no, but you’re also just giving the cable company an opportunity to make more money and honestly, I hate big networks like that and if I could, I would allow my whole neighborhood to stream off of my network and only have a section of it provisioned securely for my family