
[UPDATED 05 Jan 2020] There is now an official Ubuntu 18.04 LTS image for Raspberry Pi 3. Install the arm64 image following instructions on the Ubuntu Wiki. You can then install MongoDB 4.2.x per this blog entry. NOTE that I still recommend adding swap space – as mentioned below.
[UPDATED 03 Nov 2019] Looking to run MongoDB 4.2.x on a Raspberry Pi 4? Check out this post. It is also possible to run Ubuntu 18.04.2 (ARM 64-bit) directly on the Raspberry Pi 3. You can read more about this and get SD Card images from the Ubuntu wiki. I am using the server version of “Ubuntu Classic” as opposed to the new “Snappy Ubuntu Core.” MongoDB has an official release of MongoDB 4.0.6 4.0.11 for “Ubuntu 16.04 Linux 64-bit ARM 64.” I will show how to install this version of MongoDB under Ubuntu 18.04 on the Raspberry Pi 3B+. These instructions should work for any version of R-Pi 3 but I have only tested on my Raspberry Pi 3B+.