Answer
$\sin^2\theta; \space \space 2\cos^2\theta; \space \space2sin^2\theta$
Work Step by Step
$\cos{2\theta}$ can be written in $3$ different forms, namely:
(1) $\cos{2\theta)} = \cos^2\theta-\sin^2\theta$
(2) $\cos{2\theta)} = 2\cos^2\theta-1$
(3) $\cos{2\theta)} = 1-2\sin^2\theta$
Thus, the missing expressions in the given statement, in order of appearance, are:
$\sin^2\theta; \space \space 2\cos^2\theta; \space \space2sin^2\theta$
