SLA — Service Level Agreement. It’s more law-related. It’s about contracts.
SLI — Service Level Indicator. It’s some parameter that should be measured and which should be kept in some range.
SLO — Service Level Objective. It says how often SLI could fail. Like SLI should be true for 99.9% of the time.
Video from Google’s engineer (in Russian) — https://youtu.be/avdS6aIs4yI
Another look on that:
An SLI is a service level indicator; a carefully defined quantitative measure of some aspect of the level of service that is provided. e.g Request Latency, Error Rate.
An SLO is a service level objective; a target value or range of values for a service level that is measured by an SLI. e.g. 95% of /homescreen calls will complete in less than 100 ms.
SLAs are service level agreements; an explicit or implicit contract with stakeholders that includes consequences of meeting (or missing) the SLOs they contain.