A Raspberry Pi 3B+ has only 1GB of ram and this official Bitcoin Core page shows that minimum requirement is 2GB ram and 200 GB storage with 100MB/s read and write speed.
But I see a lot of articles where people installing full node on a Raspberry Pi 3 or even older models. Also I see some tutorials where they install full node and run testnet daemon and they attach 500GB+ external HD. Why would they need an external HD when testnet blockchain size is about 8GB.