Apparently there are three existing potions to get 300% xp but I can only find two of them (Rapid Mind, Ancient Knowledge) so where can the third be found ?

I don't think there is a 3rd, it's two 300% potions, recruit a friend which = 300% and then BoA gear which to many is 40% and then + 10 more % if you manage to acquire the BoA rings.

Oh I understand now ! I thought the 3rd +300% was with a potion. Thank you man :)