At day 3 of the dendritic cell (DC) generation, I can observe both loosely attached and non-adherent cells. Do these cell populations show different phenotypes?

No. At day 3 of the dendritic cell (DC) generation protocol, there should be no significant phenotypic difference between the non-adherent cells and the loosely attached dendritic cells.
If you would like to assess the phenotype at this stage, gently rinse the culture surface to detach a portion of the loosely attached cells and combine them with a representative fraction of the non-adherent cells for downstream analysis. This approach provides a sample that reflects the overall dendritic cell population present in the culture at day 3.
Note: Loosely attached cells at this stage are typically considered part of the developing dendritic cell population and can be analyzed together with the non-adherent fraction.
Further reading: For comprehensive guidance on monocyte-derived dendritic cell (moDC) generation, maturation, morphology, and phenotypic characterization, please see our Application Note Generation of Monocyte-Derived Dendritic Cells (moDCs).