Create an entity, calculated from others

My first post and my first scirmish to customise my use of HA. (I have a few tasmota devices (sonoff/wemos d1) talking to HA (installed on pi4) and a lovelace UI.
My goal is a graph on lovelace that plots a calculated value based on two temperatures and a flow sensor sent by tasmota on a wemos d1. I can get the sensor values to HA as entities but I just can’t figure out how to process these with a little algebra. Id like to define a new entity made of three others (I think!)
Not after a step by step answer, just someone to help open the door on HA for me to go and learn.
I’ve not written any yaml yet and don’t exactly understand it. Maybe this is where I’m failing. If there is a solution that doesn’t need me to mess about with that, that would be great.

The more I fumble about in HA the more I think I need to just bite the bullet and learn how yaml works?