Can I try 3-4 TUMS (calcium carbonate) for severe leg cramps while on Prednisone? I do take multivitamin & a Magnesium, Zinc, Calcium supplement. Have OA. Any harm?

Possibly. TUMs (calcium carbonate) may affect the absorption of other medications. Also if they have calcium in them you do not want to go above 1,500 mg of calcium a day because of the risk of kidney stones. Also you said you have OA and we typically do not use Prednisone for that condition. Please make certain of your diagnosis.
Need lab. I am no sure why you are on prednisone but with severe leg cramps you should have lab work. Tums (calcium carbonate) have calcium but that is not likely to help. .