Can i take Doxycycline, Metronidazole, and Amoxicillin?

Your problem may and may not be drug interaction, and not having a PDR on hand, I won't guess on that, but it is not the first place my mind went reading your problem. You have a more complicated problem if you are self treating - if you get too much antibiotic at theraputic blood levels, you can kill off the bacteria in your gut that helps digest your food, giving you the absolute worst case of diarhea, and it is possible to shut your kidneys down as well. So, if you are under a doctor's care, he ought to know that, and should have planned your 3rd concurrent dose with that in mind if he knows about the other two, and should be asked the question so you know for sure he thought about it. If not, keep in mind that there are darned good reasons these things are only offered by prescription.

First of all you mention your problem and after this ask for medicines. Apart from that Doxycycline and Amoxicillin are medicines of same group so you can take any one them. Metronidzole is an anaerobic medicine. Withour knowing your problem none can suggest you.