Am, C major and D Major are all chords from the key of G major / Em.
It's a D minor in the key of C
With the chords you mentioned, Em pentatonic will work fine but so will the Am pentatonic
It's worth noting that in any minor key you can play 3 different pentatonic minor scales and still be playing notes in the same key but with a different flavour.
For example in the key of Em you could play Em pentatonic, Am pentatonic or Bm pentatonic. Each different scale just picks 5 different notes from the E minor scale.
It's even easier if you just think of it as always having the option to play a basic minor pentatonic shape at the root, 5 frets above or 7 frets above