3. A city's phone numbers are being increased from six to seven digits, which can add $\qquad$ more dialing combinations.

将上面的文本翻译成英文，请保留源文本的换行和格式，直接输出翻译结果。
3. $10^{7}-10^{6}$.

Six-digit numbers can form $10^{6}$ dialings, seven-digit numbers can form $10^{7}$ dialings. After the upgrade, $10^{7}-10^{6}$ more dialings can be added.