Discussion about Heroes 4.

...when Nature + Scouting reaches 15+15, and then I level up Combat to 15, will the class change? Or will I remain a Bard until some skill reaches 16? And in any case, since Tactics is planned to be more than 15, when it crosses that threshold, which skill will it combine with? With Scouting because it's the starting skill, or with the last skill leveled to 15 (in this case, Combat)? ...

When the 3rd skill reaches the level of the two previous ones, the class won't change; it will increase by one level and will likely combine with the first. But with frequent class changes, I can't tell you what the connection will be. Moreover, with a large number of skills, when leveling up, it might offer something completely different from what you plan – for example, Summoning, Seafaring, and Resistance, or Charm – none of the skills you need, so choose what you want.