Does this mean that a character needs specialized knowledge in all three modes (land/sea/air) in order to ride in those modes, or just the modes they didn't choose at novice?If they are a novice in mount skill, they may only ride on their chosen mode with novice skill. It's not until competence that they are able to branch out. Once they reach competence, they will need to acquire specialized pieces of knowledge (which don't count toward the skill total), in order to ride in that mode.
For example: Fred chooses land riding and rides a Scalvwing. When he reaches competent, does he need to get extra knowledges in both land and air to continue riding his Scalvwing, or does his chosen mode mean he can ride it competently on the ground, but he needs to get air riding knowledges if he wants to ride it in flight too?