A three-alarm fire sparked by a short-circuited radiant floor heater caused more than $5 million in damage Wednesday morning to a posh three-unit condominium building on Commonwealth Avenue near Boston’s Public Garden.

“I’d say more than anything here the loss of any property or furniture isn’t really that ­important,” said John Walsh, the owner and chief executive of the Elizabeth Grady salon and spa chain, who was taking a shower inside his third-floor condominium when he heard the fire alarm blaring at about 9:18 a.m.