This paper investigates the extended main-sequence turnoffs (eMSTOs) and stellar populations of three clusters with different ages using Gaia Data Release 2 (DR2) data in detail. The colour-magnitude diagrams (CMDs) of three Galactic clusters show broad main sequences, eMSTOs, blue stragglers, and red giant clumps. We explore the effects of age spread, binaries, and rotating stars on the morphology of the entire CMD, paying particular attention to the effects of binary stars and rotating stars on eMSTOs. Some synthetic CMDs are built on the basis of the advanced stellar population synthesis (ASPS) model to study the observed CMDs and search for the best-fitting stellar population models. The age of the youngest stars, age spread, metallicity, colour excess, distance modulus, binary fraction, and rotating star fraction of clusters are determined. The results show that both binaries and rotating stars are responsible for the eMSTO of intermediate-age cluster LP 585, but binary stars affect more than rotating stars on the eMSTO morphologies of the youngest (M11) and oldest (NGC 6819) clusters in our samples. It suggests that M11 (similar to 0.3 Gyr) is possibly a composite stellar population of rotating and binary stars, while clusters LP 585 (similar to 1.1 Gyr) and NGC 6819 (similar to 2.0 Gyr) are simple stellar populations of rotating and binary stars.