If you look up single speed conversions you'll that either you need a horizontal dropout so the wheel can slide back enough to tension the chain, or you need some other chain tensioning method, like an eccentric hub or bb or a pseudo-derailleur chain tensioner if you're running a single rear cog and single chainring. Though I suppose you could keep the triple crankset and front derailleur, and keep the rear derailleur as a chain tensioner.