replacement for old capacitor
In my comment I missed that you mentioned one of the pins is marked NC. That clears things up completely.

A capacitor is a two terminal device. On the one you have the third pin is marked N/C meaning, not connected (or no connect). On very large capacitors, adding a third terminal was done to give some mechanical support to reduce stress on the component leads caused by vibration. The third lead does nothing electrically speaking.

Basically what you need to do is find a standard two terminal \$4700\mathrm{uF}\$ capacitor rated for at least \$50\mathrm{V}\$ which has a similar spacing between the two important leads (the ones not marked N/C). Any will do, though ones with lower ESR ratings will perform better at smoothing out supply rails (in sensitive circuits the lower ESR could cause issues, but given it is an electrolytic capacitor, you probably won't find one that large with an ESR low enough to cause problems).

When you solder in the new component, be sure to use some potting compound to help provide the mechanical support that the third leg used to. Hot glue can be used, but unless you can get it to go through some holes somewhere nearby, or wrap over other components, it won't stay attached for long. You are better using something like silicone epoxy. If you use the search tool here, there are some questions about potting compounds.