Dioxygenase that catalyzes the conversion of the modified genomic base 5-methylcytosine (5mC) into 5-hydroxymethylcytosine (5hmC) and plays a key role in epigenetic chromatin reprogramming in the zygote following fertilization. Also mediates subsequent conversion of 5hmC into 5-formylcytosine (5fC), and conversion of 5fC to 5-carboxylcytosine (5caC). Conversion of 5mC into 5hmC, 5fC and 5caC probably constitutes the first step in cytosine demethylation. Selectively binds to the promoter region of target genes and contributes to regulate the expression of numerous developmental genes. In zygotes, DNA demethylation occurs selectively in the paternal pronucleus before the first cell division, while the adjacent maternal pronucleus and certain paternally-imprinted loci are protected from this process. Participates in DNA demethylation in the paternal pronucleus by mediating conversion of 5mC into 5hmC, 5fC and 5caC. Does not mediate DNA demethylation of maternal pronucleus because of the presence of DPPA3/PGC7 on maternal chromatin that prevents TET3-binding to chromatin. In addition to its role in DNA demethylation, also involved in the recruitment of the O-GlcNAc transferase OGT to CpG-rich transcription start sites of active genes, thereby promoting histone H2B GlcNAcylation by OGT. Binds preferentially to DNA containing cytidine-phosphate-guanosine (CpG) dinucleotides over CpH (H=A, T, and C), hemimethylated-CpG and hemimethylated-hydroxymethyl-CpG (By similarity). 2-oxoglutarate + a 5-methyl-2'-deoxycytidine in DNA + O2 = a 5-hydroxymethyl-2'-deoxycytidine in DNA + CO2 + succinate Binds 1 Fe(2+) ion per subunit. The zinc ions have a structural role. Interacts with HCFC1 and OGT. At the zygotic stage, localizes in the male and female pronucleus, while it localizes to the cytoplasm at other preimplantation stages (PubMed:21892189, PubMed:28930672). Binds to the promoter of target genes, close to the transcription start site (PubMed:26774490). Highly expressed in oocytes and zygotes. Not expressed in embryonic stem cells (ES cells). Expressed maternally. Expressed at high levels in oocytes and zygotes, with rapidly declining levels at the two-cell stage. The CXXC-type zinc-finger domain mediates binding to DNA sequences containing unmethylated cytosine or 5-carboxylcytosine in 5'-CCG-3' DNA sequence motifs (PubMed:26774490). It mediates binding to CpG-DNA (By similarity). Neonatal lethality. A germline-specific conditional knockout produces females that are normal in growth and morphology but display much reduced fecundity in terms of the frequency of successful pregnancy per mating and the litter size. No 5hmC signal is detected in the late male pronuclei of zygotes collected from the conditional knockout females mated with wild-type males. In contrast, deletion of Tet3 from the male germ cells does not seem to affect the change in 5hmC and 5mC. Belongs to the TET family. Subsequent steps in cytosine demethylation are subject to discussion. According to a first model cytosine demethylation occurs through deamination of 5hmC into 5-hydroxymethyluracil (5hmU) and subsequent replacement by unmethylated cytosine by the base excision repair system. According to another model, cytosine demethylation is rather mediated via conversion of 5hmC into 5fC and 5caC, followed by excision by TDG. Truncated N-terminus. Contaminating sequence. Potential poly-A sequence.