Answer
An SLI is a measurement: the share of requests that succeeded within 300 ms. An SLO is the internal target for it: 99.9% over 30 days. An SLA is a contract with customers, with refunds or penalties when broken. The SLA is set looser than the SLO, so the team reacts before money is at stake.
Why
Good SLIs measure what users see, ideally at the load balancer or client, not CPU or memory. The gap between the SLO and 100% is the error budget.