We don't have synonyms for sanmai yet — it may be a rare, specialized, newly coined, or misspelled word. The closest thesaurus entries we have are sancai, sanai and sanman.
(n) :
(uncountable) A construction method for Japanese-style blade steels, usually sandwiching a hardenable steel in a taco of flexible steel, that is forge-welded together, leaving a hardenable edge steel and flexible spine steel, for single-edged blades.
(n) :
(countable) A steel sandwich made by this method, before and after being welded.
(n) :
(uncountable) A construction method for Japanese-style blade steels, usually sandwiching a hardenable steel in a taco of flexible steel, that is forge-welded together, leaving a hardenable edge steel and flexible spine steel, for single-edged blades.
(n) :
(countable) A steel sandwich made by this method, before and after being welded.