Hyperpolarization-activated ion channel that are permeable to sodium and potassium ions, with an about 3:1 preference for potassium ions (PubMed:16043489). Contributes to the native pacemaker currents in heart (If) and in neurons (Ih). In particular, plays a pivotal role in maintaining excitability and promoting rhythmic burst firing within hypothalamic nuclei. Exerts a significant influence on the configuration of the cardiac action potential waveform. Does not appear to play a prominent role in the processing of acute, neuropathic, or inflammatory pain (By similarity). {ECO:0000250|UniProtKB:O88705, ECO:0000269|PubMed:16043489}. This is the function of ENSG00000143630 (HCN3, hyperpolarization activated cyclic nucleotide gated potassium channel 3).