\documentclass{article}
\begin{document}

\textbf{bf}, \textit{it}, \textbf{\textit{bfit}}.

%\mathbf{bf}, \mathit{it}, \mathbf{\mathit{bfit}}.

\[ a+\textbf{bf}, b+\textit{it}, c+\textbf{\textit{bfit}} \]
\[ a+\mathbf{bf}, b+\mathit{it}, c+\mathbf{\mathit{bfit}} \]

\end{document}
