<TL;DR>Ignore the note after the / and just play normal C G and D chords</TL;DR>
Generally slash chords mean that you play the chord before the slash but with the note after the slash as the bass note. In these cases where the second note is part of the chord it specifies the inversion.
On a ukulele you usually will just ignore the part after the slash since with only four strings it can be difficult to play specific inversions with out jumping all around the fretboard, especially if you have high-G. On a low-G your standard open C chord is the C/G inversion (the open 4th string is G) and you'll need to go up to the 11th fret to get the F# on the 4th string.
Unless it's part of a walking bass line it probably won't matter.
If you have a bass player they'll play note after the slash. If not, get a bass player