It is not a blanket permit which allows any businessman, academician or professional to shave any time during the Three Weeks. Nowhere is it suggested that since it is permitted for a businessman to shave under extenuating circumstances, he may therefore dispense with the prohibition altogether. Accordingly, a businessman who goes on vacation or is away from his office for several day over the weekend or a legal holiday, is not permitted to shave during that time. Some people who became accustomed to shaving during their years of employment continue to do so even upon retirement. This is not permitted.
Sephardic communities have different customs.
See The Weekly Halachah Discussion, pg. 423-428 for the details. These rules apply to the days of Sefiras ha-Omer as well. The Three Weeks period includes another period of more intensive mourning, called the Nine Days. We are discussing the laws of the Three Weeks only, not the special, more stringent, halachos of the Nine Days.