One thing you can try is to wrap your repeater in a UL and put LI tags inside the Repeater's ItemTemplate. CSS styles for each LI would be inline, with a width of 33%. That would give you 3 per line.
<ul class="addresses">
<asp:Repeater ID="AddressList" runat="server">
<ItemTemplate>
<li class="address">
<asp:Literal ID="Address" runat="server" Text='<%#Container.DataItem.ToString().Trim() == ","? "Please fill in your primary address.":Container.DataItem.ToString() %>'></asp:Literal>
<br /><asp:Literal ID="Phone" runat="server" Text='<%# getInfo(AlwaysConvert.ToInt(Eval("AddressId"))) %>'></asp:Literal><br /><br />
</li>
</ItemTemplate>
</asp:Repeater>
</ul>
And the CSS:
ul.addresses { width: 100%; margin: 0; padding: 0; }
ul.addresses li.address { width: 33%; display: inline; }