I've taken 3 doses of mucinex (guaifenesin) sinus max severe congestion relief today I'm over due for my next dose. Can I take Claritin -D instead now?

Yes. A good idea for self-medicating cold/sinus symptoms is to list your symptoms and present the list to the pharmacist who can guide you through the litany of options. If self-medication fails to improve symptoms or they worsen, best to consult a physician. That said, if you are otherwise healthy (and have no serious heart or blood pressure related condition) can now take the claritin-D.