According to the standard international English grammar, the correct 3rd Conditional of the sentence “If travel counselors are available, ask to speak with them.” would be rephrased as
A
As long as travel counselors have been available, you could ask to speak with them.
B
Unless travel counselors had been available, you could ask to speak with them.
C
Provided that travel counselors were available, you should ask to speak with them.
D
Had travel counselors been available, you should have asked to speak with them.
E
If travel counselors would have been available, you should have asked to speak with them.