Almost always functions as a noun-modifier. Key collocations:
三叉路口 (three-way road junction / Y-intersection),
三叉戟 (sānchājǐ, trident — Poseidon's weapon, also the name of British Aerospace's airliner / Trident missile),
三叉神经 (sānchā shénjīng, trigeminal nerve — the cranial nerve responsible for face sensation, named for its three branches).
