I took zyrtec during my pregnancies.
Most of the medications you might take for allergies are generally safe during pregnancy, especially during the third trimester and you are close to that. Among these drugs are what are called “Non-sedating antihistamines” and inhaled nasal steroids. What you should do first, however before taking any meds, is to check with your obstetrician and get his/her recommendations.
The other thing of course is complete avoidance of any environmental allergens, to the extent that that is possible.
Good luck