With that I mean the first, fourth and fifth chord in a (mostly major I think) key.
With major keys, the 1, 4, and 5 are major, rest is minor except for 7. You can usually count up from the root note (the 1) to find the rest. For example in the key of C. C is 1, D is 2, E is 3, F is 4, G is 5, A is 6, B is 7 and then you return to the root.
As I said before, 1, 4 and 5 are major, the rest is minor except for 7 which is diminished. You can fill in the math. 1 is Cmajor, 2 is Dminor, 3 is Eminor, 4 is Fmajor, 5 is Gmajor, 6 is Aminor and 7 is Bdiminished.
For that you need to know what notes are in a key. The funny thing actually is that the key of Aminor contains the exact same notes as C major. You actually already mentioned the key of C major in your progression from C to F to G. Those are the 1, 4 and 5 chords in C major.
A minor is the realitive minor in the key of C major. A minor is the 6th chord in the key of C major. So if A minor is the 1/root of a key, the 4 and 5 are Dminor and E minor. But since you are playing in a minor key the sound will be very different.
I'm sure Guitartricks has a tutorial on this to explain this in a better way.