The calculated distance (air line) between Frenštát pod Radhoštěm and Třinec is approximately 22 miles respectively 36 kilometers.
By car or train, the actual journey to Třinec is certainly longer, as only the direct route (as the crow flies) between Frenštát pod Radhoštěm and Třinec has been calculated here.
Air line (approximately)
36 kilometers
19 nautical miles