If you mean you only want item 3, then then don't use a loop at all:
var third = Model.dynamicFieldInfo.DynamicFields_List[2];
Otherwise, use a counter outside the loop:
int count = 0;
foreach (var item in Model.dynamicFieldInfo.DynamicFields_List)
{
if (++count == 3)
{
... do something with it ...
count = 0
}
}