Hi I have a Dell optiplex 790 (not the SFF version) that has an i7 2600 and I was wondering if I could change the processor to an i3 8100 without having any problems with the motherboard or any other hardware.
No. The generational gap is too big. They use different sockets. At most with any Intel CPU, you can only use two generations on a single socket. So right now you are using 2nd gen, the most you can upgrade to is 3rd gen.
Further more, 8th gen uses DDR4 RAM and your current CPU means you have DDR3