\[\boxed{\mathbf{3.}\mathbf{229}}\]
\[\textbf{а)}\ 37 = 37 + a\]
\[a = 37 - 37\]
\[a = 0.\]
\[\textbf{б)}\ 37 - a = 37\]
\[a = 37 - 37\]
\[a = 0.\]
\[\textbf{в)}\ a - 37 = 37\]
\[a = 37 + 37\]
\[a = 74.\]
\[\textbf{г)}\ 0 = 37 - a\]
\[a = 37.\ \]