There is a strange problem that comes with owning a successful investment.The better it does, the harder it can become to buy more.That may be how some investors feel about Singapore’s three big banks these few weeks.DBS Group Holdings Ltd (SGX: D05), Oversea-Chinese Banking Corporation Limited (SGX: O39) and United Overseas Bank Limited (SGX: U11) have enjoyed a remarkable run.Their businesses are performing well. Profits remain strong. Dividends are rising.And their share prices have climbed to, or close ...