Third derivative would be 2[##2sec^2 x tan^2 x +sec^4x]##

If y= tan x

y’ = ##sec^2 x##

y”= 2secx secx tanx= ##2sec^2 xtanx##

y ”’ = 2##[2sec x sec x tan x tan x+ sec^2 x sec^2 x}##

= 2[##2sec^2 x tan^2 x +sec^4x]##