AMC 12 2018a Problem 3

How many ways can a student schedule $3$ mathematics courses -- algebra, geometry, and number theory -- in a $6$-period day if no two mathematics courses can be taken in consecutive periods? (What courses the student takes during the other $3$ periods is of no concern here.)

$\textbf{(A) }3 \textbf{(B) }6 \textbf{(C) }12 \textbf{(D) }18 \textbf{(E) }24$