I would agree with some type of laddering approach but disagree with what has been stated above.
Insurance companies typically have "banding" (where the cost per $1,000 of death benefit is reduced once you reach certain face amounts). Therefore, in many cases, the higher the coverage amount, the lower the cost per $1,000 of death benefit. By purchasing multiple policies from multiple companies, you are also incurring additional annual policy fees. If you purchase smaller death benefit (face) amounts, that policy fee is higher relative to the total annual premium.
Banner Life's (William Penn in New York) Term policy allows you to add term riders to the base policy. As a result, you can purchase your coverage from one carrier and eliminate additional policy fees. So, using the example above, you could purchase a $500,000 30-Year Level Premium Term policy from Banner Life, with Level Premium Term Riders for $500,000 each for 10-Years and 20-Years.
When you are dealing with a total coverage amount of $1,500,000 (which is relatively small) it really does not pay to spread out the risk or pay additional monies for this "protection" in the unlikely event that something happens to one of the carriers.
Here is an article that I wrote on how to shop for term life insurance. Hopefully, you will find it informative.
http://www.physicianfinancialservices.com/files/7760/OPM Term Life Article.pdf