Try this:
On a separate sheet to where you want the validation to occur, type in A B and C in separate cells. Select those cells, click on Insert-Name- Define and enter a name in the top box (say myList) and click on 'Add'. Go onto the sheet where you want the data to be validated, select the cells and click on Data-Validation, change the 'Allow' to List and in the Source box enter =myList. HTH.