Short answer: All three report health coverage for taxes. 1095-A comes from the ACA marketplace (needed to reconcile subsidies), 1095-B comes from insurers/small plans showing you had coverage, and 1095-C comes from large employers (50+) documenting their offer of coverage.
At tax time you may receive one of these forms depending on your coverage. Form 1095-A is sent by the ACA marketplace to people with subsidized exchange coverage; you need it to reconcile premium tax credits on Form 8962. Form 1095-B is issued by insurers, small self-funded employers, or government programs simply to show you had minimum essential coverage. Form 1095-C comes from Applicable Large Employers (generally employers with 50 or more full-time employees, including full-time-equivalents) and documents the coverage they offered each full-time employee, used to administer the employer mandate. Only the 1095-A is typically needed to complete your return; the others are informational.