Heroes are allowed more than one skill. However, every time you add a skill (up to 3) your hero has a chance to forget a skill.
So in essence it is another chance mechanic making it extremely difficult and costly to get the 3 skills you desire.
Wait until you upgrade. Suppose you have the 3 skills you like but you find a rank 2 version of one. If you try to learn it, you have a 1 in 3 chance of getting it into the right slot. If you miss and because higher ranks don't stack with lower ranks you just lost the effect of 2 skills.
Safe mechanic is to stick with a single skill or multiple common lower level ones.