Small conductance calcium-activated potassium channel that mediates the voltage-independent transmembrane transfer of potassium across the cell membrane through a constitutive interaction with calmodulin which binds the intracellular calcium allowing its opening (PubMed:12808432, PubMed:20562108, PubMed:31155282, PubMed:36502918). The current is characterized by a voltage-independent activation, an intracellular calcium concentration increase-dependent activation and a single-channel conductance of 10 picosiemens (PubMed:12808432, PubMed:20562108, PubMed:31155282, PubMed:36502918). Also presents an inwardly rectifying current, thus reducing its already small outward conductance of potassium ions, which is particularly the case when the membrane potential displays positive values, above + 20 mV (PubMed:12808432). Activation is followed by membrane hyperpolarization. Thought to regulate neuronal excitability by contributing to the slow component of synaptic afterhyperpolarization (By similarity). {ECO:0000250|UniProtKB:P70605, ECO:0000269|PubMed:12808432, ECO:0000269|PubMed:20562108, ECO:0000269|PubMed:31155282, ECO:0000269|PubMed:36502918}. [Isoform 3]: Does not function as a small conductance calcium-activated potassium channel (Probable). Selectively suppresses endogenous KCNN3 currents, in a dominant-negative fashion by decreasing the abundance of functional channels in the plasma membrane, possibly by selectively coassembling with and sequestering native KCNN3 protein in intracellular compartments (PubMed:12808432). This dominant inhibitory effect extends to other members of the SK subfamily (PubMed:12808432). {ECO:0000269|PubMed:12808432, ECO:0000305|PubMed:12808432}. This is the function of KCNN3 (potassium calcium-activated channel subfamily N member 3, ENSG00000143603).