What causes cats to vomit white foam?

3 out of our 5 cats began vomiting white foam, several days apart. House is cat-proof, no strings/cleaning products/plants/etc. First cat went to ER, no fever but severe gastric distress. Xray showed enlarged colon and shrunken kidneys. Put him on IV fluids, took to regular Vet who administered iodine test in stomach. Definite blockage, unknown if foriegn body/tumors or severe gastritis. 2 days later, second cat vomited white foam/smacking lips. Got to Vet (third Vet) in two hours, slight fever/normal Xray/good red and white blood counts/negative feline leukemia and AIDS but put in hospital as precaution and on IV fluids SHE WILL NOT EAT, still in hospital. Her Xray did show a hairball. Now, 24 hrs later, third cat is vomiting white foam. Three different Vets cannot tell me what is happening, I have paid $2,800 so far and cats are still sick and vomiting white foam. Please help if you can, they are indoor only.