Is it safe to take plain robitussin(guaifenesin only), ibuprofen and doxycycline all at the same time?

As long as you can: safely take these medications individually, yes, it's safe to take them together. Be aware, however, that both doxycycline and ibuprofen can upset your stomach or give you heartburn. Doxycycline might be less effective when take with dairy products but otherwise all these meds can be taken with food. This might help prevent stomach upset. Dr. Anne.