How to make formulae take equal vertical space in the align environment?

if you do not want it globally changed put the complete formula into {...}

\documentclass{article}
\usepackage{amsmath}
\begin{document}

\savebox\strutbox{$\vphantom{\dfrac11}$}
\begin{align}
  f_1(x) &= \frac{15x}{3} \\
  f_2(x) &= 3x + 5 \\
  f_3(x) &= 4x + 13
\end{align}

\end{document}

I don't know of an automatic way to do this, but in a particular case, you can use \vphantom to effect this.

\documentclass{article}
\usepackage{amsmath}
\begin{document}
\begin{align}
f_1(x) &= \frac{15x}{3} \\
f_2(x) &= \vphantom{\frac11}3x + 5 \\
f_3(x) &= \vphantom{\frac11}4x + 13
\end{align}
\end{document}

I can offer a variant of TH's answer that produces a bit less vertical space, but it is adjusted to your particular situation: Only the second line needs a bit of additional depth. The advantage of this more special adjustment is that it doesn't produce additional white space below the display.

\documentclass{article}
\usepackage{amsmath}
\begin{document}
Here's some text just to fill the line sufficiently
\begin{align}
f_1(x) &= \frac{15x}{3} \\
f_2(x) &= 3x + 5 \vphantom{\smash[t]{\frac11}} \\
f_3(x) &= 4x + 13
\end{align}
Here's some text just to fill the line sufficiently
\end{document}