Sam Smith and their boyfriend of more than three years, fashion designer Christian Cowan, are engaged , Page Six can exclusively reveal. The couple, who attended the 2026 Met Gala together on Monday, were overheard talking about their engagement at the Mark Hotel before walking the red carpet. Cowan accessorized his black couture Erté-inspired ensemble — which he designed along with the musician’s matching getup — with a massive square yellow diamond ring on that finger, which we’re told is by Cartier. Sam Smith and their boyfriend of more than three years, fashion designer Christian Cowan (pictured above at the 2026 Met Gala), are engaged, Page Six can exclusively reveal. Getty Images The couple, who attended the gala together on Monday, were overheard talking about their engagement at the Mark Hotel before walking the red carpet. REUTERS “From what I understand, it was a private engagement,” our hotel lobby spy told us.…