I'm sorry to hear that you have multiple pets with itchy skin and scabs.
It would be extremely uncommon to have multiple, unrelated pets of different species with inhaled allergens or food allergies in one home. It's possible, just not very likely.
There are very few pests that commonly affect multiple species.
Fleas are the usual culprit in a home with multiple itchy animals. If you are using an over the counter flea preventative its possible you have a resistant flea population. And if you haven't treated your home it is possible that fleas that are hatching out from eggs or larvae are continuing to develop and bite your pets. Only 5% of the total flea population are adults living on your pets. The rest in the environment. By only treating the adults on your pets it can take a long time to get rid of fleas.
Flea allergy dermatitis is horrible. In allergic animals it only takes one bite a month for them to itch like crazy. If you don't see any fleas you might not have a huge population and if they are very allergic they may be removing any the evidence with their excess grooming. Ideally you need to apply topical flea control every 3 weeks during in an allergic animal. Many over the counter products have a problem with fleas being resistant and some can be down right dangerous. I never recommend any products that Hartz or Sargent's make. I recommend Advantage II, Advantix, Advantage Multi, Frontline Plus every 3 weeks for at least 4 months for indoor cats and all year round for cats that go outside or if there are dogs in the home. All pets in the house must be treated as if you don't the nonallergic ones serve as a flea reservoir for the allergic one and you'll never solve your problem. Another option is the relatively new Seresto flea and tick collar. This collar is the only proven effective collar and works extremely well for 6-8 months.
It may help to treat areas where the animal spend a lot of time with an area treatment like Siphotrol Plus to get rid of developing eggs and larvae.
If they seem to have more dander then usual and the dander seems to be especially bad along their back then we need to worry about a microscopic parasite called Cheyletiella or "walking dander". This is a highly contagious skin surface mite that stimulates skin cell production and causes extreme itchiness. Your veterinarian can take a sample of the dander and look at it under the microscope to look for mites. If they are present all mammal pets in the home should be treated and a thorough clean-up of the environment should be done to get rid of the mites, and their eggs. The most effective treatments for dogs and cats are topical Selamectin (Revolution) or oral Ivermectin which are prescription products. In light infestations we can use a flea shampoo followed up with flea control topicals containing Fipronil (like Frontline Plus) every 3 weeks. We must be sure to also clean up the environment, including kennels, floors and bedding as this mite can survive in the environment and reinfestation occurs if a thorough cleaning isn't done. You may need to use flea area treatment products like Siphotrol Plus II premise spray.
Your veterinarian can also test them for another microscopic mite called Sarcoptes which causes very itchy skin, especially on the ears, bottom of the chest, elbows and hocks. Treatment is with Revolution (selamectin) or ivermectin.
I would treat all mammal pets in the home. This mite doesn't survive long in the environment so simply washing bedding in very hot water should be enough to treat the environment.
Your veterinarian can also culture their lesions for a contagious fungal infection called ringworm.
In the meantime to control itchy skin symptoms in dogs or cats you can try:
1) Benadryl (diphenhydramine only, the combination products with decongestants and acetaminophen are toxic to cats and dogs) at 1mg to 2mg per pound of body weight every 8 hours. That's a half of 25mg tablet every 8 hours for a pet that is 8 to 15 pounds.
2) Chlorpheniramine at 2mg to 4mg once or twice daily.
3) Zyrtec (Cetirizine hydrochloride) at a dose of 5 mg per cat or dog given orally every 24 hours. (Make sure it is NOT the formulation with a decongestant (such as Zyrtec-D) because cats and dogs cannot tolerate decongestants.)
Combined with high doses of omega-3 fatty acids antihistamines work to relieve the itchiness. I like 3V caps or Derm Caps. I recommend a dose based upon the EPA portion (eicosapentanoic acid) of the supplement as if we do that the rest of the supplement will be properly balanced. Give 20mg of EPA per pound of body weight per day. For example an 8 pound animal could take 160mg of EPA per day. Together antihistamines and omega-3 fatty acids work synergistically. These should help reduce the itch. Be aware that antihistamines can cause drowsiness or hyperactivity which should resolve with continued use.
Best of luck with your pets please let me know if you have any further questions.