Ribonuclease (RNase) III involved in RNA-mediated post-transcriptional gene silencing (PTGS). Involved in the processing of repeat-associated small interfering RNAs (ra-siRNAs, derived from heterochromatin and DNA repeats such as transposons) by cleaving small dsRNAs into 24 nucleotide ra-siRNAs. Plays a role in antiviral RNA silencing. Involved in the production of viral siRNAs derived from the cabbage leaf curl virus (CaLCuV) and tobacco rattle virus (TRV). Targeted by the viral silencing suppressor (VSR) protein 2b of the cucumber mosaic virus (CMV) that inactivates DCL3 function in RNA silencing. Acts redundantly with DICER-LIKE 1 (DCL1) to promote flowering via repression of FLOWERING LOCUS C (FLC). Does not seem to be involved in microRNAs (miRNAs) processing. Interacts with DRB2 and DRB5. Belongs to the helicase family. Dicer subfamily.