I can imagine the three players being in a huff. It is difficult to judge 'from a distance', not having seen what actually transpired but consider some possibilities:
- it sounds like you were trying to railroad the players/PC's into going into a sub-plot, if the PC's felt that they have no business / stake in whether one or the other lord becomes king (maybe they are all just a bad / good as the other as far as they know), it is perfectly legit to want to finish the mission going from a to b. If the 'raising an orc army' was something discussed earlier as being something on their mind, it could have come over as being a delaying tactic in stead of offering an interesting sub-plot.
- If among the three are long time players (don't know about the other two), it could also be seen as being unkind of you as a DM to side with the minority, and telling them you have not prepared for them, only for the sub-plot, only reinforces to them you were forcing them into that subplot (you did not have anyhting else prepared!), and to them you let down old-time friends, the majority to boot!
All the best advice here is the same: talk to the players out of the game itself.
- are they bored with the campaign in general / their own PC's in specific?
- are there Out Of Game issues between the two/three player groups, which bubbled up In Game to cause this split?
- are they upset / p'd off with the process of how things went down last session?
As for 're-introducing' the PC's back into the game, some good ideas were given here as well (the most elegant being have them meet up with the orcs and somehow find out they are tied into the whole kidnapping plot anyhoo), but check first with the players. If for some reason (besides childish 'I dont wanna' type reasons) they dislike this campaign thread, then simply abandon it. Let the other PC's quickly find the son, bring him back to the father (or have them encounter a patrol already looking for the son, and so the PC's need not do more then point out where he is held, and the PC's can be on their way to meet up with the others).
Hope this helps. Main issue is: settle things OOG first, before choosing how to solve thing IG.