すでにMaven2がインストールされているところにMaven3をインストールする手順です。

brew unlink maven2
brew install maven

Maven2に戻す方法

brew unlink maven
brew link --overwrite maven2

$ mvn -v
Apache Maven 2.2.1 (r801777; 2009-08-07 04:16:01+0900)
Java version: 1.8.0_31
Java home: /Library/Java/JavaVirtualMachines/jdk1.8.0_31.jdk/Contents/Home/jre
Default locale: ja_JP, platform encoding: UTF-8
OS name: "mac os x" version: "10.10.1" arch: "x86_64" Family: "mac"

Maven3に戻す方法

brew unlink maven2
brew link --overwrite maven

$ mvn -v
Apache Maven 3.2.5 (12a6b3acb947671f09b81f49094c53f426d8cea1; 2014-12-15T02:29:23+09:00)
Maven home: /usr/local/Cellar/maven/3.2.5/libexec
Java version: 1.8.0_31, vendor: Oracle Corporation
Java home: /Library/Java/JavaVirtualMachines/jdk1.8.0_31.jdk/Contents/Home/jre
Default locale: ja_JP, platform encoding: UTF-8
OS name: "mac os x", version: "10.10.1", arch: "x86_64", family: "mac"