If they only differ by the contents of the form, then put everything else into the Master page, and the form alone into each Content Page.
I'd leave the Submit method for each form in it's Content page's code-behind, because these will be independent of each other.
You could have them all link to the same javascript validation file, for neatness, and call different validation methods
<asp:ScriptManager ID="ScriptManager1" runat="server">
<Scripts>
<asp:ScriptReference Path="Validation.js" />
</Scripts>
</asp:ScriptManager>
and have each form's Submit button call its own method, e.g.
<asp:Button ID="btnSave" runat="server" OnClientClick="ValidateFormA" OnClick="Save" Text="Save" />