Personally I wouldn't buy a mini computer for this use case. Using the mini computer form factor has zero advantages for CCTV usage and a lot of disadvantages (price, lack of HDs, heat management, expandability, etc).
I'm not as concerned about the heat management (3 cameras isn't going to be a lot of CPU load on the machine), but it certainly could be an issue - especially if you expand your needs. However the fact that you'll have to use an SSD drive for recording footage is a huge issue IMHO. Most SSD drives are not rated for 24/7 use (and certainly the one you would get with this machine is not going to be rated for this). The fact that you only want to keep 1 day of information on the SSD actually makes this worse because not only will you have constant writes occurring, you'll be transferring data off the SSD regularly as well. So while using an SSD drive for CCTV usage will technically work, I would expect it to completely fail much sooner than it is suppose to. There is a reason why we recommend using a hard drive designed for 24/7 read/writes (many times referred to as server or CCTV grade).
You would be much better served by buying a used SFF computer off EBay with something like an i5-8400 chip. They cost about $200, have room for a real hard drive, have better heat management, and have room for expansion as well.
