Temperature fluctuations, is it normal? I have an Anycubic i3 Mega-S and when I'm printing something (especially when it's hotter) the temperature increases and decreases by around 3 degrees during the print. Is this normal?

I was worried this is a loose thermistor and if it comes out during a print my house will probably catch fire! I have no idea how to tighten it though.
All printers will have some fluctuation and it's not a concern. This is similar to the fluctuation you'll see in your home temperature around the thermostat setting.  There are a couple reasons. One is that the feedback loop (thermistor to control board to heater to extruder block) will always have some lag. Another is that most systems have built-in "hysteresis," i.e. set the "turn off heat" a couple degrees above the "turn on heat" thresholds. This avoids "chatter" from on to off right at the setpoint.
Some thermistors (e.g. my AnetA8 clone) can be held in place with a setscrew. If yours doesn't have a similar capability, use Kapton tape to hold it in. This tape is designed for high temperature operations so it won't age or collapse, and it has good holding strength.