Hello, i have been an owner of 2 raspberry pi’s and i want to know is it possible to host a server on a pi 3?
Good day! 
You absolutely can run a server on a Raspberry, since server specs are irrelevant (every client renders vehicles for themselves, the server is only sending players’ positions)! The only requirement, really, is “a low latency and enough bandwidth, about 4MByte/s combined in/out”. See recommended distros on the BeamMP wiki. You will also need to build from source, since your architecture isn’t x86_64. Please refer to these forums and its wiki for more info!
Now, if you want to combine your two Pi’s into a cluster, you will need to have two things: a Compute Module 4 IO Board and a Pi Zero. Could be a bit costly though than renting a cloud server.
I wish you all the best, you have inspired me to set a server on a Pi of my own. <3
Good luck and be well! 