What are the standards for creating key pairs?

Let's say I have 3 servers that I need access to, and I switch between 3 clients (PC, laptop, phone).

• The easiest solution would be to make 1 key pair, put the private key on every client and the public key on every server (not as safe)
• I could make one pair for every server, and have 3 keys on each client
• I could make one pair for every client, and have 3 keys on each server
• Or I could have 9 pairs, 1 for each combo (seems a little unnecessary)

I suppose in my case it doesn't matter too much, they're just personal servers. But I still like doing things right (and I'm curious), so is there a standard way it's done? I'm assuming it's one of the middle two, but I'm not sure which.